Most experts recommend drinking at least a cup (four to 10 ounces) of water every 15 minutes of exercise.
Drink about 8 oz. every 20 minutes while you run.

Research shows that your heart rate increases eight beats per minute for every liter of sweat lost during exercise.

You can become dehydrated in as little as 30 minutes of exercise depending upon the environment and your intensity.

Please don't count coffee, beer or caffeinated or alcohol containing drinks in your daily tally of fluids. They act as diuretics and will cause an overall fluid loss.
Note: Caffeine weakens the detrusor muscles in the bladder, which provokes the need to urinate, but does not affect diuretic hormones the way alcolyl does.

Immediately following exercise the muscles are able to replenish glycogen stores at a much higher level than a few hours later, Try a protein-carb mix to more rapidly replenish your glycogen stores after a long run. But don't forget your overall fluid replacement needs should be met with a sports drink or water. so a sports drink or a fluid source such as water with another source of carbohydrates for glycogen replenishment would be fine. Don't forget that if you are using Sport Gels or Bars for carbohydrates drink 4 oz to 8 oz of water with them to aid your digestion and assist in their absorption. There is some evidence that a protein-carbohydrate drink may assist in glycogen replacement better than carbohydrates alone.

In 2004, the Food and Nutrition Board of the nonprofit Institute for Medicine (part of the National Academy of Sciences) released a report on the recommended intake of water (and some minerals). While exact water requirements were not specified, general daily recommendations for women were set at 2.7L (91 fl. oz.) and 3.7L (125 fl. oz.) for men. This figure includes "total water"Ñwater derived from all beverages and foods. About 20% of a human's daily water intake comes from foods. Exercise intensity and high temperatures clearly will increase a person's daily water needs.

Excessive water intake can actually be life-threatening. But by far the greater risk facing active outdoor athletes is under-hydration and dehydration.

Play it smart. Step up your awareness of body signals when active outdoors, particularly if the intensity, temperature or elevation is higher than normal for you. Drink fluids regularly and generously, even before your thirst-alert mechanism kicks in. Using a hydration system gives you an advantage. Because fluids are so easy to access, you'll drink more often and as a result perform better.