One day you will not be able to do this, today is not that day.
I like that one...
Have got a US Marines top with the "Pain is just weakness leaving the body..." motto on it.
I like the Roger Bannister one (although it's not a phrase per se)...
"Every morning in Africa, a gazelle wakes up. It knows it must outrun the fastest lion or it will be killed. Every morning in Africa, a lion wakes up. It knows it must run faster than the slowest gazelle, or it will starve. It doesn't matter whether you're a lion or a gazelle--when the sun comes up, you'd better be running."