What does not destroy us make us stronger (a misquote from Friedrich Nietzsche) has been part of the signature block on my personal email for many years. Originally I thought it meant that strength came from victory, from not being defeated, from conquering things. But now, as an ultra runner, I see things differently.
Ultra running is mostly about being mentally tough, you need to be reasonably well trained, but you must be able to push ahead when you are cold and tired on a Scottish mountainside or hot and tired in a desert (I prefer the desert). It is the spirit and the will to keep going that matters. At times the body will crumble and no amount of willpower will get you through. That is not a problem as the body will mend and you can prepare it better for the next time.
It is when the spirit buckles that things get interesting. Allow too much damage and you will be permanently weakened, even more and you will be destroyed. When things go wrong it is important to know when to persist, when to modify what you are doing and when to turn away and try something new. I have seen people keep on trying when they were never going to succeed and be broken, crawling away to live a lesser life. It is not victory that makes you stronger, but how you handle the challenge no matter how it unfolds.
I push myself close to the boundary, others choose not to get so close and some take it right to the edge. It is a personal choice how close you get testing the boundaries of what is possible. But to not try is to be condemned to a life of mediocracy.