There's good and bad news about running in the heat. But by gradually preparing yourself for increased temperatures and taking action from the beginning of hot weather runs, you'll get a welcome dose of the good news. You'll learn how to hydrate yourself, what to wear, and when and how much your body can take in hot weather, all of which will help you recover faster and run better than others of your ability on hot days. While even the most heat-trained runners won't run as fast on hot days as on cold ones, they won't slow down as much nor will they feel as much discomfort.
Many runners get into serious trouble even on moderately warm days without knowing it.
Anyone who has heart disease risk factors or suspicions of these should talk to a doctor trained in exercise before continuing. Until the temperature rises to about 65 degrees F, most runners don't notice much heat buildup, even though it is already putting extra burdens on the system. But soon after that, if the temperature is above about 62 degrees F, you're suddenly hot and sweating. On runs and especially races under those conditions, most runners have to force themselves to slow down. It's just too easy to start faster than you should when it's 60 to 69 degrees F because it feels cool at first.