Although a lot depends on whether you are a first time marathon runner or a seasoned marathoner .Ideally you should eat around 0.8 gm to 1gm protein per kg of your body weight. Body gets energy from carbohydrates but after it is exhausted, protein is broken down to release energy .Generally Carbs are hot favourites still Proteins should make one fourth part of your diet. They help in repair, renewal of cells, recovery from injury etc. Meat, eggs, dairy products are rich sources of proteins. You can try different combination of protein diets before your practice session and check how your body metabolism reacts to them.