A runner draws confidence from a lot of places: past workouts, a full season of uninterrupted training, race times, training with those who have faster PRs than you, etc. A large part of being mentally tough is being confident that you can warp the messages coming in to your brain, and override them to push through the pain.
This confidence is built the longer you run; it's a snowball effect. As with all other rules of running, confidence hinges upon consistency—you have to prove consistently to yourself that you can push through the pain. There are margins for error and, just like bad races, there will be days where you don't do a great job of running and overriding the pain messages.
—Caitlin Chock