Obviously, you'll want to be sure to have plenty of fluid available throughout your hot workouts. Since heat increases both the sweat rate and thirst, plan to carry or have access to a significantly greater volume of fluid than you would need for a workout of equivalent length in cooler weather.
Be aware, however, that staying hydrated has a very limited capacity to keep the core body temperature from increasing. A much more effective way to stay cool is to slow down. Listen to your body when training in the heat and go as slowly as necessary to remain relatively comfortable. Exertional heat illness is relatively rare because the nervous system ensures that we feel lousy before we're in real danger; don't try to override this self-protective mechanism.
Finally, when preparing to train in very hot weather it's a good idea to plan routes that allow you to get indoors quickly if necessary. For example, you might do an eight-mile run comprising eight one-mile loops around your neighborhood. It's a little dull, maybe, but it's better to be bored and safe than entertained and at risk.
More: Fluid Facts for Athletes
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