Many athletes are seeking new and novel ways of pushing their limits, and the challenge of running back-to-back races is certainly one way to test the boundaries.
If it comes easy, if it doesn’t require extraordinary effort, you’re not pushing hard enough. It’s supposed to hurt like hell.
Unless you're not pushing yourself, you're not living to the fullest. You can't be afraid to fail, but unless you fail, you haven't pushed hard enough."