You can't train the same way every day and expect to make gains. The same goes for your diet. A 150-pound runner may require only 2,500 calories on a rest day but take in 3,700 calories, with lots more carbohydrates, on endurance days. "The main points," says Boston nutritionist Nancy Clark, author of Nancy Clark's Sports Nutrition Guidebook, "are to eat enough carbs to fuel big workouts and to time meals appropriately before and after." Use this plan to ensure you get the calories you need.