This article seems to be all about the definition of "everything" in "everything is possible if you put your mind to it". For the author "everything" includes swimming faster than Michael Phelps even if you didn't start swimming at an age of 7. That's a tough one.

The saying is much more useful if you consider only the things that are possible given the current circumstances. I may not become as fast as Michael Phelps, but I can definitely become as fast as I can possibly be given my current situation. If you look at it that way, "Just do it" applies and it's a good motto to live by.