Don't Stop when you are Tired. Stop When You are Done.

It hurts, but that’s all it does. The most difficult part of the training is training your mind. You build calluses on your feet to endure the road. You build calluses on your mind to endure the pain. There’s only one way to do that. You have to get out there and run.

Everybody comes to a point in their life when they want to quit. But it's what you do at that moment that determines who you are.