> 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.