Carbohydrates are fuel for your body. The exact number of carbohydrates you burn per mile depends upon your fitness level, experience level, pace and running economy, but most athletes average around 100 calories per mile with about 75% of those calories coming from carbohydrates. If regular training is a part of your routine consider ditching the low carb diet for optimal performance even if weight loss is one of your fitness goals.
A typical racer's training diet is one that is around 65% carbohydrates – 25% protein – 10% fat. Most athletes believe that a carb is a carb. The truth is that all carbs are not created equal. Some supply energy very quickly while others provide a slow, steady stream of energy. For that reason you should be eating low GI (Glycemic Index) carbs during your training periods to regulate peaks and troughs in blood glucose levels. Low GI carbs are also best for general health and weight maintenance.
The addition of some fats along with some low GI foods will also help keep your blood glucose stable. An example would be two pieces of whole grain toast with butter and a banana. Avoid high GI foods on race morning because it can cause your blood glucose to fall and you will end up using a very high percentage of liver glucose for energy early in the race. Keep the nutrition tailored to your training. With proper attention to GI in the week leading up to your event as well as on race day you'll have a topped-off tank and then some.