I would guess a few things are a bit off in your analysis:
1. You probably burn less than 1/2 the calories from fat.
2. You probably have less than 2000 calories available. I've read that it is closer to 1500, but either way, some of that is stored as glycogen in muscles that aren't being used for cycling, and thus isn't really accessible.
3. You're performance will suffer long before you run out of glycogen completely.