The half marathon (21.1 km) is a test of endurance and speed. But when your half marathon training schedule lands in the middle of a warm season, the challenge intensifies. The physiological cost of running 15km in 25°C heat is vastly different from running it in 10°C.
To reach the finish line strong, you need to adapt your training logic to the environment.
Understanding Cardiac Drift
In summer, you might notice your heart rate creeping up steadily during a long run, even if your pace remains constant or slows down. This is called “cardiac drift.”
The Cause: As you sweat, your blood volume decreases. Meanwhile, your body diverts blood to the skin to cool you down. This leaves less blood for your muscles, forcing your heart to pump faster to maintain the same output.
The Fix: Ignore the GPS pace. If your training programme calls for an “easy run,” stick to your easy heart rate zone, even if that means running 30 seconds slower per kilometre. You are still getting the same aerobic benefit.
Structuring the Long Run
The weekend long run is the cornerstone of any 21K plan. In summer, this run requires logistical planning.
Loop Routes: Instead of running 10km out and 10km back, try running smaller 5km loops near your house or car. This allows you to set up a personal aid station with cold water, ice, and electrolytes that you can access every 30 minutes.
Pre-Cooling: Research suggests that lowering your core temperature before you run can improve performance. Drinking an ice slurry or draping a cold towel around your neck before starting can delay the onset of heat fatigue.
Nutrition and Electrolytes
For a half marathon, glycogen stores are key. However, in hot weather, gastric emptying (how fast food leaves your stomach) can slow down, leading to sloshing and nausea.
Liquid Calories: In high heat, you might find it harder to chew energy bars. Switch to isotonic gels or liquid carbohydrate mixes that provide both energy and hydration in one go.
Salt: If you finish a run and your skin tastes salty or you see white streaks on your shirt, you are a “salty sweater.” You need to be aggressive with salt tabs or electrolyte powders to prevent cramping.
The Benefit of “Heat Doping”
There is a silver lining to the struggle. Training in the heat increases your blood plasma volume and improves your sweat rate. When race day arrives—assuming the race is in cooler weather or starts early—you will often feel like a superhero. The effort required to run your goal pace will feel significantly lower. This effect is so powerful that scientists sometimes call it “poor man’s altitude training.”
Recovery
Post-run recovery is crucial when the heat has drained you. Get out of your wet clothes immediately to prevent chafing and saddle sores. Rehydrate with a cold protein shake to lower your core temp and kickstart muscle repair.
