Most humans can work their way up to a mile without too much drama. Working your way up to 500? That seems to be more drama than most people are interested in participating in.

I'm sure a lot of it is genetic, but, probably just as much of it is mental. Few years ago I could barely run a mile. Now, I can run twenty plus. But you have to want it. We were born to run but like other things, lose that ability when we don't use it.