the body metabolises both stored glycogen (from the liver and muscles) and fat for energy during your race.
there are physiological limits to how much glycogen you can store in your body (this equates to about 90mins of high intensity exercise). There is a MUCH higher limit to how much fat you can store.
if properly nourished before the race, and assuming you have no medical conditions that interrupt normal metabolism, when you run at low relative intensity your body will metabolise fat for about half of the energy requirements of your muscles. At higher intensities, the energy demand increases, and it's more efficient to use glycogen (the metabolic path is quicker and cheaper) to satisfy those demands.
An elite distance runner will have trained to a degree where s/he can draw on fat stores for a higher proportion of energy requirements for longer. There is also some evidence that small amounts of caffeine ingested before exercise can increase the rate of fat metabolism.
So in summary, this explains why some people can run an entire marathon on nothing but a bit of water to rehydrate themselves, as they're using a mixture of fat and carbs from the outset to supply their muscles with energy, and they never fully deplete those glycogen stores. Unless you've starved yourself, you don't need any additional carbs during workouts up to about 90 mins in length, and that's assuming you burn glycogen only, and burn no fat at all.
Edited: 06/02/2013 at 13:42
