I've read that high level body builders will actually eat candy after a workout for the same effect. Also, ultra marathons will have mountain dew at aid stations for that immediate boost.
Yeah there is interesting "math" you can do. But, for most people, your liver stores about 1200-1500 calories of glycogen. You have some in all of your muscles, etc. Marathon is right at the distance you can run and stay under "the wall". In fact, that is a key part of pacing. Finding your race pace to basically operate on glycogen for all but the last bit of it (1-2 miles) where you can just push/suffer through on will power. If you go out too hard, you burn glycogen too fast, hit the wall at 6 miles out... that is the suck. 6 miles can be an actual eternity when you hit the wall.
