An hour before you start your run, try to drink 16 ounces of water or other non-caffeinated fluid. This is especially important if you're running in the morning, since you'll most likely wake up dehydrated. Stop drinking at that point, so that you can void extra fluids and prevent having to stop to go to the bathroom during your run. To make sure you're hydrated before you start running, you can drink another 4 to 8 ounces right before you start.
Nutrition and Hydration Guide for Long Distance Running
Sports Nutrition Mistakes
What to Do the Day Before a Half or Full Marathon