RELATED: Fueling for Your First Marathon. Exactly how much carbohydrate is optimal for you depends on how you run. My general rule of thumb is this: Start at a baseline of 3 grams of carbohydrate per kilogram you weigh (1 lb = 2.2 kg) per day. Add 1 g/kg/day for every 20 miles per week you run. So if you run 40 miles per week, aim for about 5 grams of carbs per kilogram you weigh daily.