Many beginning runners underestimate the importance of nutrition, for both their running performance and their overall health. What and when you eat before, during, and after your runs has a huge effect on your performance and recovery.
Try to eat a light snack or meal about 1 1/2 to 2 hours before a run. Choose something high in carbohydrates and lower in fat, fiber, and protein. Some examples of good pre-workout fuel include: a bagel with peanut butter; a banana and an energy bar; or a bowl of cold cereal with a cup of milk. To avoid gastrointestinal distress, stay away from rich, high-fiber, and high-fat foods.
If you're running more than 90 minutes, you need to replace some of the calories you're burning. You can get carbs on the run through sports drinks or solid foods they are easily digested, such as as energy gels, bars, and even sports jelly beans designed for long-distance runners. A basic rule of thumb is that you should be taking in about 100 calories after about an hour of running and then another 100 calories every 40-45 minutes after that.
Replenish energy as quickly as possible after a workout. Studies have shown that muscles are most receptive to rebuilding glycogen (stored glucose) stores within the first 30 minutes after exercise. If you eat soon after your workout, you can minimize muscle stiffness and soreness. You'll want to consume primarily carbs, but don't ignore protein. A good rule of thumb for post-workout food is a ratio of 1 gram of protein to 3 grams of carbs. A peanut butter and jelly sandwich, a fruit and yogurt smoothie, and chocolate milk are examples of good post-run snacks.
Don't follow a low-carb diet when training. You need a certain amount of carbohydrates in your diet because they're a runner's most important source of fuel.
More: Nutrition and Hydration for Runners
Healthy Snacks for Runners