According to webmd, Drink Up — Before, During and After- A good guideline to use when preparing for an outdoor workout, whether it’s walking, running, biking, or tennis, is to drink about two cups of fluid two hours before the activity. That helps make sure you are well-hydrated before you ever go outdoors.
Then, during the activity, try to drink 4-6 ounces every 15-20 minutes to keep your muscles well-hydrated. If you are planning an hour-long walk or gym workout, fill a water bottle with about 16 ounces (2 cups) and take it with you. Last, drink up after you’re finished with your exercise. If you really want to be precise, weigh yourself before you start exercising and again when you are finished. For each pound of water weight you lose, drink 20 ounces of fluid.