The best advice for this is to obey your body’s signs of thirst. Everyone’s hydration demands are different however there are some guidelines. At fluid stations along the race route, drink no more than one cup of fluid every 20 minutes. Just drink based on your thirst level and take small sips rather than big gulps. Although it is okay to drink plain water in the early miles you should consume sports beverages after 30 minutes of continuous exertion. During your training, you figured out what works for you so now stick to it on race day.