Some people use carbohydrates better than others.
Some gain more sustained energy by using protein and fat, both of which can be stored for long-term energy supply. If carbs seem to be the thing to keep you going for the shorter, high-output session, make sure they are unrefined. This means no concentrated sugar or white flour. Stay with carbs such as fruit, whole-grain flour, pasta and breads. Here's an easy way to determine your percentage of body fat and lean body mass.