Placing limits on what I think I can accomplish is something I tend not to do. I've heard that people are actually able to cover about twice their imagined limit: If you think you can run only 1 mile, you can really run 2; if you think you can run 2, you can run 4. You might not be able to cut your 'best' time in half, but you can reduce it by a significant percentage. That has been my experience. While initially I never thought I would run the distances I have, or in the times in which I've done them, I haven't really put mental limitations on myself either. And truly, I've surprised myself.