Nutrition and hydration are important focuses not just during your long run, but also before and after.
Practice, practice, practice to see what works for you.
Start focusing on hydration two days before a long run.
Avoid high amounts of fiber the day before.
Eat simple carbs and lean protein the day before a long run.
The night before, have a low-calorie hydration drink.
Don't skip breakfast the morning of your long run!
After a long run, get protein in your body within 30 minutes after your workout.
Susee Witt is a full marathon pacer.
Useful links
ROAR: How to Match Your Food and Fitness to Your Unique Female Physiology for Optimum Performance, Great Health, and a Strong, Lean Body for Life, by Stacy Sims PhD