system
Given the following post, respond as if you're commenting on Reddit:
human
If the human body stores 2 hours worth of glycogen and considerably more than 2 hours worth of fat, why do people get tired before 2 hours of jogging?

What is the biological nature of physical exhaustion? Especially after low intensity aerobic exercize - jogging, or even walking for a long period of time?
gpt
Glycogen is mobilized quickly. Fat needs some extra processing to become available as muscle fuel.

Additionally, it's true that most people have ample reserves of "fuel", but their bodies are not prepared to mobilize and burn vast amounts of it quickly. A large part of endurance training involves changes in the body that just increase its ability to process and burn energy reserves.

An easy measure of that is the amount of oxygen burned in a given time (or the amount of CO2 released - same thing). An athlete can burn a lot of O2 per minute; a couch potato, not so much.