Heat Training for Kona: How to Prepare for the World Champs from a New Zealand Winter
The race that punishes the underprepared
Kona doesn't care how fit you are.
I've watched athletes with the engine to break 9 hours come undone on Queen K because they treated the heat like a race-week problem. It's not. The heat is a 6-month problem with a 14-day intervention at the end.
If you've qualified for the IRONMAN World Championship and you're sitting in a New Zealand winter wondering how on earth you're going to be ready for 32-degree humid lava fields, this is for you. Same applies if you're racing IM Cairns, IM Texas, or any 70.3 in conditions you don't train in.
I'm Mark Bowstead, founder of Bowstead Performance Coaching. I race professionally for years on the international circuit, finished 3rd at IRONMAN Australia with the bike course record, and have spent the last 7+ years coaching age-group athletes through races in conditions they were never built for. This is the framework I use.
Why heat is the silent killer of Ironman performance
Here's what's actually happening when your core temperature climbs in a race.
For every 100 watts you put into the pedals, your body releases roughly 300 watts as metabolic heat. In Auckland in July, that heat dissipates fine. On the Queen K at noon with 80% humidity, the gradient between your skin and the air collapses. Sweat stops evaporating. Your body has nowhere to dump the heat.
So it does the only thing it can: it diverts blood to your skin to cool you, and away from your working muscles. Less oxygen to the legs. More cardiovascular drift. Higher heart rate at the same power. And critically, your body shifts toward burning carbohydrate instead of fat, because anaerobic glycolysis kicks in earlier under thermal stress.
That last point is the real performance killer. Your finite glycogen stores get torched in the first 4 hours, and the marathon turns into a survival shuffle.
Heat training fixes this. Not partially. Fundamentally.
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What heat acclimation actually does to your body
Two big adaptations matter for Ironman.
Plasma volume expansion. Within 3 to 5 days of repeated heat exposure, your blood volume expands by 6.5% to 15%. More fluid in the system means a higher stroke volume, lower heart rate at the same effort, and more capacity to perfuse both your muscles and your skin without one robbing the other.
Sweat response. You start sweating earlier, sweat more efficiently, and lose less sodium per litre of sweat. Your body becomes a better thermostat.
The downstream effects are huge. Threshold heart rate drops by 10 to 15 bpm. Lactate at any given power drops. Fat oxidation is preserved at higher intensities. You stop bleeding glycogen.
This is why heat-adapted athletes don't just race better in the heat. They race better in the cold too. There's good evidence that heat training produces an 8% improvement in time-trial performance even in temperate conditions, because plasma volume expansion is essentially a poor person's altitude camp.
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How long does heat acclimation take?
The science is actually pretty clear here, and most age-groupers get it wrong.
Most adaptations happen in the first 5 to 9 days of repeated thermal exposure. By day 10 to 14, you're at near full adaptation. Beyond 14 days, you get diminishing returns and rising injury risk from accumulated stress.
So the sweet spot is 10 to 14 days of heat sessions, where each session elevates your core temperature above 38.5°C for at least 60 minutes.
Two windows matter for Kona:
1. A 14-day induction block 6 to 8 weeks out from the race. This is where you build the adaptation.
2. A 6 to 10 day refresher in the final 2 weeks before the race. This relocks the adaptation that has started to decay.
Adaptations decay at roughly 2.5% per day once you stop the stimulus. Skip the refresher and you'll arrive in Hawaii half-adapted at best.
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When should I start heat training for Kona?
If your race is in October, your formal 14-day induction block should sit somewhere in the August window. That gives you 6 to 8 weeks of recovery and racing-load adaptation between the induction block and the race week refresher.
What I do with my Kona athletes:
- June and July: general fitness build. No formal heat work, just 1 sauna session per week to keep the door open.
- Mid-August to early September: the 14-day induction. This is where the work happens.
- September: maintenance. 2 heat sessions per week, kept surgical.
- Final 10 days into Kona: refresher block. Lower volume, sustained thermal stimulus.
The mistake I see most often is athletes panicking in the final 3 weeks and trying to cram heat training on top of taper. That doesn't acclimate you, it cooks you.
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How do I heat train when I live in cold New Zealand?
This is the question I get most. You're in Wellington in July. It's 9 degrees and raining. How do you replicate Kona?
Three options, ranked by effectiveness.
Option 1: Post-workout sauna (the gold standard)
If you have access to a sauna, this is what works. The protocol is simple but specific.
After a key training session (not on easy days), get into a traditional sauna at 85 to 95°C for 25 to 30 minutes. No hydration in the last 10 minutes of training. No hydration during the sauna. The point is to drive your core temperature up and keep it elevated long enough to trigger the adaptation. Get out when you're genuinely uncomfortable, not when you're in distress.
Rehydrate gradually over the next 3 to 5 hours with electrolyte-rich fluid. Aim for 150% of the weight you lost during the session.
10 sessions across 10 to 14 consecutive days will get you most of the way there.
Rest intervals during the sauna block: keep at least 20 to 24 hours between sauna sessions. If you're doing twice-a-day training, sauna goes after the second session, not the first.
Option 2: Hot bath protocol
If you don't have a sauna, this works almost as well and there's solid research behind it.
After a 30 to 40 minute easy run, get into a bath at 40°C and stay submerged to your shoulders for 30 to 40 minutes. Same hydration rules apply. The key is the duration, not the peak temperature, because what you're doing is keeping core temp elevated.
Six consecutive days of this is enough to drive measurable plasma volume expansion. Add it to a normal week of training and you have a viable home protocol.
Option 3: Indoor trainer with heat
If you have a Wahoo or a Tacx in a small room, this is your friend.
Turn the heating on. Close the windows. Wear a long-sleeve top and gloves. Turn off any cooling fans. Do a 60 to 90 minute Zone 2 ride aiming to feel like the room is somewhere between 28 and 32°C. The dryer running in the next room adds humidity if you want to layer that in.
This is the worst of the three options because the thermal stimulus is hard to control, but it's free and it works if you're disciplined. Just don't try to hit hard intervals while doing it. Heat plus intensity equals injury, not adaptation.
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The Bowstead Method: how I structure heat training for Kona athletes
The framework I use sits on three principles.
Principle 1: Heat is a metabolic intervention, not a comfort exercise
Most coaches treat heat training as "getting used to feeling hot." That misses the point. The goal is to expand plasma volume, suppress the glycolytic floor, and protect fat oxidation. The discomfort is a side effect of the stimulus, not the stimulus itself.
This means I never let athletes "tough out" heat sessions at high intensity. Most heat work happens at Zone 1 to Zone 2. The thermal load does the work. Your job is to stay in the zone long enough for the adaptation to fire.
Principle 2: Heat layers onto base, never onto build
The 14-day induction block sits in the second week of a 3 to 4 week training block, not in a recovery week and not in your hardest week. You want training load steady or slightly reduced while heat stress is added on top. Compounding heat with peak training volume is how athletes wind up in the overtraining hole 4 weeks out from race day.
Principle 3: The refresher matters more than the induction
If you only had 10 days to do heat work, I'd put it in the 10 days before the race, not 8 weeks out. Adaptations decay. The race-week refresher is non-negotiable.
For my athletes, that final block looks like:
- Days -10 to -7: 30 to 40 minute sauna sessions after easy aerobic work, 4 sessions
- Days -6 to -4: travel to Kona, light training in heat, 2 sessions outdoors
- Days -3 to -1: passive heat only, no aggressive sauna, focus on hydration and acclimating to the time zone
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Three Bowstead heat sessions you can use this week
Session 1: Big Gear Heat Induction (bike, indoors)
Goal: VLamax suppression, cardiovascular stability under thermal load.
- 20 min warm-up, ramping into Zone 2 at 85 to 95 rpm
- 5 x 6 min big gear efforts at 80 to 85% of FTP, 50 to 60 rpm. Recovery 2 min easy spin between efforts.
- 45 min steady Zone 2 with all fans off, wearing one long-sleeve layer
- 10 min Zone 1 cool-down
The big gear blocks build muscular durability. The 45-minute Zone 2 block is where the heat does its work. If the heat starts winning, drop the power but finish the duration. The clock is what matters.
Session 2: Strides + Post-Run Sauna (run, outdoor + sauna)
Goal: Maintain neuromuscular speed while triggering plasma volume expansion.
- 25 min easy aerobic run, building to mid-Zone 2
- 10 x 100m build strides, starting at 80% effort and building to 95%. Walk-back recovery (60 sec) between each.
- 30 min steady Zone 2 running
- Within 15 min of finishing, 25 min sauna at 85 to 95°C. No hydration in the sauna.
- Cool down with a warm shower, then gradual rehydration over 3 hours
This is the highest-bang-for-buck session in the whole framework. Strides keep your running economy sharp during a phase where most heat protocols dull it. The sauna does the metabolic work.
Session 3: Kona Swim Durability (swim, pool)
Goal: Aerobic durability in warm water with neuromuscular touch-up.
- 500m mixed warm-up (100 swim / 50 kick / 50 drill / 100 pull)
- Main set: 4 x 800m continuous. 70% effort on the first, 75% on the second and third (focus on a firm catch), 80% on the fourth. Rest 20 sec between each 800.
- Neuromuscular block: 12 x 25m max sprint. Rest 45 sec between each.
- 200m easy cool-down
Total: 4,200m, which is just over the Ironman swim distance. The progressive 800s build aerobic durability. The 25s keep your top-end sharp without taxing the system. Rest intervals are non-negotiable here. 20 seconds between 800s, 45 seconds between sprints. Don't shortcut them.
Kona's swim is in 28°C water. This set trains your body to operate in warm water without panic.
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How heat training changes your fuelling and hydration
Three adjustments that matter.
Sodium goes up. Your sweat rate climbs and so does sodium loss. Plan for 800 to 1,200 mg of sodium per litre of fluid on race day, depending on your sweat sodium concentration. If you've never done a sweat test, get one. Best $50 you'll spend this year.
Fluid intake goes up. A typical Ironman in cool conditions might call for 500 to 750 ml/hour. Kona pushes that to 800 to 1,000 ml/hour for most athletes. Practice this in training, because absorbing that much fluid while maintaining race pace is a skill, not an accident.
Calories drop slightly. When blood is diverted to your skin, your gut absorbs less. Most athletes who try to push 90+ grams of carbs per hour in Kona end up with stomach problems. 70 to 80 grams per hour is more realistic for most age-groupers. Sports drinks are easier to absorb than gels. Keep solid food for cold-weather races.
https://www.precisionhydration.com/sweat-test/ - Check this out
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Race day cooling: do this from the start, not when you're suffering
The single biggest race execution mistake I see in hot races is reactive cooling. Athletes wait until they feel overheated to start dousing themselves. By then, they're already 30 minutes behind the deficit.
Cool from the first aid station. Every aid station, ice down the front of your trisuit, water over your head, sponges on your neck and wrists. It's not optional. It's part of the race.
On the bike, slow your effort by 5 to 10% relative to your standard hot-weather threshold for the first hour. The first half of the bike is the easiest place to lose Kona, because the heat hasn't fully caught up with you yet but your heart rate is already drifting. Hold back. You'll catch the people who didn't.
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Frequently Asked Questions
Can I heat train and race-prep at the same time?
Yes, but layer them carefully. Heat work goes onto your base aerobic sessions, not your hardest interval days. The 14-day induction block sits in the middle weeks of a build phase, not in a peak load week and not in a recovery week.
Will heat training make me slower in cold weather?
The opposite. Plasma volume expansion improves performance in temperate conditions too, with research showing roughly 8% improvements in time-trial performance after a heat block. You're not just preparing for Kona, you're getting fitter overall.
Is one sauna session per week enough to maintain heat adaptation?
Yes, once you've completed an induction block. Adaptations decay at about 2.5% per day, so 1 to 2 sauna sessions per week through the maintenance phase is enough to keep most of what you built. Stop completely and you'll lose most of it within 3 to 4 weeks.
How hot does the sauna need to be?
85 to 95°C for traditional dry saunas. Lower temperatures work but require longer durations to drive the same core temperature response. Infrared saunas don't get hot enough to drive the same plasma volume adaptation. Stick to traditional.
Can I do hot yoga instead of sauna?
It will give you some adaptation but the temperature is usually too low (38 to 42°C in studios) to drive a meaningful plasma volume response. If hot yoga is what you have access to, do 60 to 90 minute sessions and treat it as a Zone 1 aerobic session. Better than nothing. Not as good as sauna or hot bath.
What if I'm racing Cairns in June, not Kona in October?
Same framework, different timing. Cairns is also a hot race despite being in winter, and most NZ athletes underestimate it. Run the 14-day induction block in April, and a 6 to 10 day refresher in the final 2 weeks before the race.
Is heat training safe for masters athletes?
It is, with adjustments. Older athletes have a slightly reduced sweat response and a slower thermal recovery curve, so I lengthen the rest intervals between sessions and start with shorter sauna durations (15 minutes building to 25). The same adaptations occur, they just need a bit more recovery between exposures.
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The honest summary
Heat training is no longer a marginal gain. For Kona, it's the difference between racing the bike course and surviving it. The science is settled. The protocol is clear. The only thing missing for most athletes is structure.
If you've qualified for Kona and you're trying to figure out how to integrate heat training into your build, your taper, and your race week, that's exactly what I do for athletes through Bowstead Performance Coaching. 1-on-1 programming via TrainingPeaks, built around your physiology, your power and pace zones, and your actual life.
