Running is a lot about mental strength.

It wasn't obvious to me when I began running that so much of it is about mental strength.

When you've build up a reasonable amount of stamina, the biggest thing stopping you from running is the mental block. The body can run at a speed for long times but the brain wants you to stop. A lot of the time, I have to distract my brain from the actual act of running, by counting my breaths, by looking somewhere else, by focusing on what I'm listening to. Once I'm finished, the body still feels okay. But if I didn't distract my brain, I'd have stopped a long time ago.