A better way to understand the relationship of how much "fat" is metaboilzed due to exercise would be to look at what the "total amount exercise" you perform divided by the "total amount of time" you take.
It's really kind of simple: Effort/Total time = Fat to sugars ratio

To expend 1000 calories in an hour would require almost all the calories to come from glycogen and glucose. To expend 1000 calories in 4 hours, you would still need to use glucose and glycogen for "life support", but much of the muscle energy could come from "fat". Perhaps as much as 60-65%.

Now here's the part to "re-confuse" you - if you look back the first sample, where the guy uses "1000 calories per hour guy" while exercising, but no fat, well guess what - he uses "fats" during the next 3 hours to maintain and rebuild glycogen and glucose stores. So on balance, as long as you exercise, you'll either burn fats while you exercise or after you exercise.

More confusion, you do all these "energy" conversions almost all the time, unless you have a disease or are dead.

I hope you read the articles suggested, and reread them if you have to, this is good stuff to know.
http://www.brianmac.demon.co.uk/fatburn.htm