Just wanted to add a note regarding the glycogen stores of well trained runners.
According to Dr. Noakes, (in “Lore of Running”; a reference on running physiology); the average non trained individual has about 1400 calories of glycogen stored in muscles. A well trained runner can more than double that; many well trained runners store 2400-3000 calories in glycogen.
Assuming a modest weight, highly trained runner, its possible to store enough glycogen to run a marathon burning no fat and requiring no refueling.
Of course nobody runs 100% on glycogen (at marathon pace) which makes it easier to cover the distance without exhausting all carb reserves.