I also wanted to add that using the example about people without one leg running marathons is really unfounded, unless you have research to back it up. I would say that losing a leg would significantly deter people from running marathons, not encourage them. Just because you notice a few people doing it doesn't mean such a life circumstance helped them.

Consider a racist public education system that oppresses blacks, and in some way favors whites. Some blacks will still have the persistence to make it to the top, and the oppression will, in a way, motivate them to work harder. But generally speaking, such a system will discourage blacks from achieving academically.

Please don't use feel-good arguments to make fallacious points.