I just completed my 2nd Melbourne Ironman in a ketogenic state by consuming Primed low carbohydrate, high fat, moderate protein (LCHF) foods. This means I was burning my fat as fuel as opposed to glucose/sugar.
By being able to use my fat stores I was able to access 40,000 plus calories for energy. A glucose burner is limited to around 2,000 or so before they must replenish. The sugar burner then uses energy for digesting the food and as some of my Filipino buddies realized this can also lead to very unpleasant stomach issues. There were a lot of people in the medics tent after the Ironman suffering from Gastro Intestinal Distress.
It is a myth that the body and brains preferred fuel source is glucose and can only run on glucose.