Before you go for race make sure that your body is fueled properly. Running any race especially the marathon where you require lot of calories make sure you consume foods that are enriched with easily digestible simple and complex carbohydrates.Stay away from things that have a lot of fat and protein, because it does not digest fast as well and make you feel lethargic. Proper hydration is very important as it helps you to maintain proper fluid levels and thermo-regulation during the race.Based on your type of race like sprint, marathon, etc plan your diet and go ahead.