Adjusting your race goal when it's hot

The cruelest race-day mistake is treating a goal time as a promise the weather has to honor. You trained all spring in cool air, locked in a pace, and then the start line reads 78 degrees with a sticky dewpoint — and you chase the clock anyway. Heat doesn't negotiate. It slows everyone, including the elites, and forcing a cool-weather pace through it is the fast road to blowing up.

The reason is physiological: in heat, blood is diverted to the skin for cooling, leaving less for working muscles, so your heart rate climbs at any given pace and the same effort simply produces a slower time. The honest move is to expect the slowdown and price it in before the start.

A widely used rule of thumb adds air temperature and dewpoint together, then scales the expected slowdown:

• 100 or below: no adjustment.
• 121–130: expect paces about 1 to 2 percent slower.
• 141–150: about 3 to 4.5 percent slower.
• 151–160: 4.5 to 6 percent slower — an 8:30 goal becomes roughly 8:56.
• Above 180: hard racing isn't advisable; shift to effort and survival.

Two principles make this work on the day:

• Run by effort, not the watch. Use perceived exertion and heart rate as your governor. On a hot day, goal effort produces a slower split — and that's correct, not failure.
• Hydrate and cool aggressively. Take fluids early and consistently, pour water over your head and neck, and use shade where you can. The cooler you keep your core, the less the heat tax compounds late.

What to do with this: check temperature and dewpoint the morning of the race, set a realistic adjusted target, and commit to running the effort rather than the old number. A smart heat adjustment loses you seconds; ignoring the heat loses you the whole race.

Source: Maximum Performance Running