Any prolonged exercise like running, walking, cycling, or mowing the lawn can place great demands on our circulation and temperature regulation systems.
Dehydration can be a serious problem.
Not only does it reduce exercise capacity, dehydration can cause cramps, heat exhaustion, and heat stroke. If you run, don't do distances greater than 10 miles when the temperature is above 83 degrees. Drink plenty of fluids. Force yourself to drink more than you want, particularly during periods of prolonged activity. We are the only animals that will voluntary drink less fluid than we need.