system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is it more difficult to run for long periods of time when exercises like biking are just as strenuous?
gpt
It's all about practice. You use different muscle groups for different kinds of exercise. 

Running is *really* hard on your body. You're obviously using your leg muscles, but you're using a whole bunch of other muscles that you may not be aware of. Abs, arms, back, neck, they all play into it with running. 

Bicycling, you're sitting down, for one. You can also stop peddling for a bit. And you have different gears. Not to diminish from cycling, but it's not really as strenuous as running.