"...when runners trained heavily and ate low-carbohydrate (40%), they had a day-to-day decline in muscle glycogen. When the same subjects ate high-carbohydrate diets (70%), muscle glycogen replacement was nearly complete within 22 hours..."
"When subjects eat only as much food as they desire ad libitum, they often underestimate their caloric needs and fail to consume enough carbohydrate to compensate or that used during training or competition. This discrepancy between glycogen use and carbohydrate intake may explain, in part, why some runners become chronically fatigued and need 48 hours or longer to completely restore muscle glycogen."
-Inside Running: Basics of Sports Physiology, David L Costill, PhD.
MY COMMENTS: The higher the mileage, the more important this becomes. Carbohydrate intake among runners should be higher than the average diet. I think sometimes we forget that, especially during heavy marathon training.