Hydration During a Marathon

Dehydration is probably the single greatest cause of marathon-related health problems, especially among first-time marathoners. The rule of thumb is “If you wait until you're thirsty to drink, it's too late.”
Drink about eight ounces of water 20 minutes or so before the race begins. Always drink at every water stop – even early in the race – during a marathon. If you'll be out on the course for three hours or more, carry your own water bottle, too. The slower your pace, the longer between water stops.