Carbo loading can be done prior to a race. The day before is usual. But even carbs in the morning before a race is effective.

The fat burning starts when you empty the glycogen chambers, but there is a sweet spot in terms of energy output, because as other people has pointed out, you can't do high intensity work on fat.

So it makes sense as an ultra endurance athlete that you try to pack som fat before a race, because it will be effective as a slow burning resource.

Another interesting thing at the moment are the ketones supplements one can drink.