Running in the Heat: How Not to Blow Up on Race Day
June, 11 a.m., 32°C (90°F), the sun fighting the shaded start of T2. You come off the bike with a plan: hold 5:10 per km. After 800 metres your body says no. By 2 km you’re slowing down. A familiar story.
The problem isn’t weakness, it’s a bad strategy. Running in the heat follows its own rules, and ignoring them costs minutes, and with bad pacing, costs your health.
Why Heat Wrecks Your Pace
Above 28°C (82°F) your body sends more blood to the skin to cool itself. Less blood reaches the muscles. Your heart rate climbs at the same pace you’d hold on a cooler day. This isn’t a motivation problem, it’s physiology.
Research on heat and endurance performance suggests that every 5°C (9°F) above the optimal starting temperature (roughly 12 to 15°C, 54 to 59°F) can slow an average amateur’s pace by 10 to 20 seconds per kilometre. Over 10 km that’s as much as 1.5 to 3 minutes.
Strategy: Forget Pace, Run on RPE
The key mistake on a hot run is fixating on pace from a cool day. In the heat, set a target RPE (rate of perceived exertion) instead of a speed.
- For an IRONMAN 70.3 in the heat: target RPE 6 to 7/10 for the first 7 km, and let your body pick up the pace only in the final three
- For a sprint or Olympic-distance race in the heat: RPE 7 to 8/10 for the first 3 km, then see what’s left in your legs
A Garmin or COROS watch will show your heart rate, which is a better measure of effort on a hot day than pace. A heart rate above lactate threshold in the first two kilometres is close to a guarantee of a blow-up in the closing stages.
Cooling: Before, During and After
Before the Run (Pre-Cooling)
The most effective method: an ice slushie or a cold drink 10 to 15 minutes before T2. Research shows that dropping core temperature by as little as 0.5 to 0.7°C (about 1 to 1.3°F) before the run start translates into noticeably better performance over the opening kilometres.
Other options: a wet towel on your neck and wrists while you wait for T2, a cold, wet headband.
During the Run
At every aid station take water and pour it over your head, neck and wrists, not just drink it. A wet shirt or a cap with a soaked brim noticeably lowers how hot your body feels for a few minutes.
If organisers hand out sponges, take all of them. Soaked shorts or socks are not a problem, overheating is.
Hydration and Electrolytes
General rule: 150 to 250 ml every 15 to 20 minutes in high heat. Water alone isn’t enough, heavy sweating strips sodium and potassium too. Electrolyte tablets or an isotonic drink at aid stations aren’t optional, they’re necessary in races above 25°C (77°F).
Warning sign: you stop sweating even though your body is hot. That’s a sign of dehydration or heat stroke, get off the course immediately.
Pacing: The Negative Split in the Heat
In normal conditions a negative split, the second half faster than the first, is a strategy for advanced athletes. In the heat it should be the default even for amateurs.
A starting plan for 10 km above 28°C (82°F):
- km 1 to 3: 15 to 20 seconds per km slower than your target pace
- km 4 to 7: watch your heart rate, hold RPE steady
- km 8 to 10: if you have anything left, pick up the pace
Simple arithmetic: start too fast and you lose 2 to 4 minutes in the second half. Start sensibly and you gain 1 to 2 minutes at the end.
Heat Adaptation, If You Have Time
The body adapts to heat training within 10 to 14 days of regular exposure. You don’t need to train through the middle of the day right away, moving a few sessions to the 10 a.m. to 2 p.m. window for two weeks before a hot race is enough. The effect: better thermoregulation, a smaller heart-rate rise in heat, lower perceived effort.
A 20-minute sauna session after training works too, the same heat-adaptation effect without running in full sun.