Simply nutritionally speaking, make sure you are consuming enough carbohydrates to keep your muscles fueled.  What "enough" means varies from person  depending on several factors including your body weight.  But a simple answer is try to get 30-60 grams of carbohydrates per hour of running...this can come from gels and/or sports drinks, bananas, potatoes, sports bars, or whatever carb source you can tolerate.  Most gels have around 25 grams of carbs per gel packet.  Look at the ingredients label for carb amount and try some things out during training.  Fluid and sodium intake are important also.  Hope that helps.