Drink plenty of fluids to prevent dehydration. Drink even if you do not feel thirsty. An average 175 lb. male loses approximately 34 fl. oz. of fluid per hour of exercise. For each 34 fl. oz. lost, it is recommended to drink 40 oz. of water.
If you feel overheated, decrease your intensity or stop. Also take steps to cool yourself including: walking in a cool, shady place while on a hiking trail or applying a cooling towel or accessory around your neck.