Endurance and the Body and Mind: General George S. Patton via @claudebacon

You've got to drive the body to the last inch of energy, and then go on. You gain nothing by just going up to where you are exhausted.
The body will only build and grow to fit the demands which the mind makes. If all you do is exercise until the body is tired, the body will get lazy and stop a bit shorter every time.
You have to go to the point of exhaustion and go on. That way, the body will figure out, "We've got to build up more body strength if that crazy mind is going to drive this hard."
If you always quit when you are merely tire, you will not gain. Once you let the body tell the mind when to quit, you are whipped for sure. You cannot gain listening to the body.
We can become stronger. We only use about half the available strength of our bodies and less than that of our minds!
We can always take one more step! When we are on the attack, we can always go one more mile.
--General George S. Patton, Jr