Exercising on a fasted state
It's much more complex than what you describe.
Circulating glucose will be used first, but this only lasts for a few minutes.
Food in the stomach does not materially contribute to blood glucose; it must
enter the intestines to be well absorbed. In any case, it's unlikely that
glucose absorbed from a recent meal will precisely match the requirements of
exercise.
If there is a deficit, glycogen stores in the liver and muscles are converted
to glucose. Typically there's about 2000 kcal stored in this way in a person
who is well fed. That can last all day in a sedentary person even while
fasting, and it can last for several hours in a person who is exercising. Fat
won't be burned at any significant rate until the glucose and glycogen are
used up. Even then, it takes a while to ramp up fat metabolism and to
accustom the brain to a change in nutrition, and during the interim expensive
glucogenesis from muscle protein is undertaken (always present to some extent
when fasting with no glycogen stores). And so on.
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