Here are approximates of how far you can get from the different carb sources, if these were the only fuel sources available.
(In reality, you always use a combination of whatever fuel sources that are available, not just a single source.)
These are approximates based on a 80 kg man who runs at a pace that expends 80 kilojoules of energy per minute:
9 miles in 72 minutes = 1200 calories = muscle glycogen storage (in legs only)
3 miles in 24 minutes = 400 calories = liver glycogen storage
0.3 miles in 2 1/2 minutes = 40 calories = blood glucose*
(*the value for blood glucose includes the glucose content of substrates and extracellular fluid)
The entire amount of free glucose in the blood provides only a 40 kcal energy reserve, which is enough to maintain this activity for only about 2.5 minutes. The rest of it comes from stored glucose (glycogen) or fat metabolism.
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