Having tried SOF selection myself (once, didn't get selected / quit before it was over), I can say that the selections are almost 100% designed to weed out people on their willpower, not physical strength. The people that aren't cut out physically, tend to be weeded out on the very first day.

And even if you get past the immediate pain stage, sooner or later you'll ask yourself "is this what I really want to do the next [N] years?".