I've been running and weight training for over 25 years. Before doing any intense cardio workout (like running or a road race), I believe you should start drinking water several hours in advance, but not immediately before. For example, start drinking water at least two to three hours ahead. Some of the best runners I know, drink water all day long, but stop an hour before their workout or race. You should be completely hydrated well before you start.

In the past, many popular guidelines suggested eight, ten or even more glasses of water per day, but modern studies show there was never much scientific evidence to support this. Today it's recommended that you simply drink when thirsty. However if you're thirsty, it can also mean you're already dehydrated. Before running, I usually drink water in advance, even it I'm not particularly thirsty.

Should you drink water during a workout? When weight training (which is largely anaerobic exercise) it's probably okay to drink some water during your workout, especially if you're sweating a lot and training for a long time. However when running short or middle distances like 3-5 miles (5-8 kilometers), I would advise not drinking any water until the workout or road race is complete. This is true for longer runs, like 10 miles (16 kilometers) too. In addition to possibly causing stomach problems (like cramps), it takes time for your body to process the water. Of course there are exceptions! For example, I can remember a couple college cross country meets where they passed out water during the race on an unusually hot, humid day in August. Another exception would be a marathon where you're running for several hours.