yeah, I've seen that slogan/adage/statement floating around for a while now. Usually associated with a gym. I think I saw a USMC shirt with it on the back. It's a simple and fairly all purpose message.
In one case It was painted on the wall in a tattoo/piercing studio (they used the longer form: "This physical pain is merely the sensation of weakness leaving my body, this pain shall pass and I will be stronger when it does").
It's a great sort of mantra really.