> most people really believe they physically cannot (say) run

Do they? I would expect that when you "physically cannot run", it means you are collapsed in a pile on the ground unable to stand or move. People might say things like "I cannot run another step", but that is not the same as actually believing it. That said, unless you are very fit and very young, it might not be a good idea to get yourself to the point where you actually cannot run anymore. Unless there is a tiger chasing you or something.

I have fallen over on the ground after 20 miles of running in both legs my quads and hamstrings were cramping. I got up and ran another 10 miles. Didn't have any choice, either run 5 miles back to the check-in table or 10 miles to finish the race.

A cramp is not being at your limits. A good stretch and your ready to continue.

This https://www.youtube.com/watch?v=CKWWRS9CpTY is being at your limits. But I'm willing to bet that a tiger would make her body remember that maaaybe there is actually (a lot) more energy left to keep going.

Was there a tiger? :)

Without getting philosophical about the definition of "belief," the point I took away was that people have a hell of a lot more left than they normally admit to themselves, even when the bar is merely "I'd be injured" rather than "I literally cannot." This may be more true for cardio than weights.