Hydrate, but not too much. The rise of recreational runners in marathons has led to longer finishing times. For some of the slower runners, hydrating at every aid station can result in drinking more than they are sweating, Stewart says. That can induce a life-threatening electrolyte imbalance called hyponatremia, he says, noting that it's important to distinguish this condition from heat exhaustion by getting screened at a medical tent.
Make sure you're acclimated to the heat. If marathon day is hotter than you're accustomed to, consider skipping it, Roberts says. Even if you've trained in the summer heat, you've likely lost your acclimatization in the cooler fall weather. It only takes two to four weeks to de-acclimate – "kind of like money, hard to get, easy to lose," he says.
Listen to your body. If you feel pressure or pain in your chest, don't run the marathon, Roberts says. And if you start to feel sick or strange while you're running, stop. "It's OK to quit," he says. "There's always another marathon."