There's this piece I've read before that is pretty good. Most people just quote the end part but I think it's best to read the entire piece so you get a better idea of the message.
It's a Bruce Lee quote. Enjoy
Bruce had me up to three miles a day, really at a good pace. We'd run the three miles in twenty-one or twenty-two minutes. Just under eight minutes a mile So this morning he said to me, "We're going to go five." I said, "Bruce, I can't go five. I'm a helluva lot older than you are, and I can't do five." He said, "When we get to three, we'll shift gears and it's only two more and you'll do it." I said, "Okay, hell, I'll go for it." So we get to three, we go into the fourth mile and I'm okay for three or four minutes, and then I really begin to give out. I'm tired, my heart's pounding. I can't go any more and so I say to him, "Bruce, if I run any more," -- and we're still running -- "if i run any more I'm liable to have a heart attack and die." He said, "Then die." It made me so mad that I went the full five miles. Afterward I went to the shower and then I wanted to talk to him about it. I said, you know, "Why did you say that?"
He said, "Because you might as well be dead. Seriously, if you always put limits on what you can do, physical or anything else, it'll spread over into the rest of your life. It'll spread limits into your work, into your morality, into your entire being. There are no limits. There are plateaus, but you must not stay there, you must go beyond them. If it kills you, it kills you. A man must constantly exceed his level."
