This was a mantra both my brothers used while going through Marine Corps bootcamp. You don't have to get to the end of the 50-mile-hike. Just take one more step. I always remember this when facing my own seemingly impossible tasks. Mental or physical.

I do something similar when running. When I feel like stopping and walking the rest of the route I'll pick a landmark up ahead, like a telephone pole or a mailbox, and say to myself, "I'll just run to that." Once I get to the landmark though, I feel I can still go on a bit so I pick another landmark up ahead that I'll run to. And so on.