Sometimes I amaze myself and find that I'm smarter than I think I am. Case-in-point: I got a 90 on the physical chemistry exam (yes, it's as bad as it sounds) that I was stressing about. To study I read over my notes, looked at the problem sets, and read the book. We had a 5 hour break between class and the exam and everyone was cramming. Except for me. I went home, exercised, and then made dinner. I figured if I didn't know it by then, I wasn't going to learn it in five hours.
I think this idea translates well to running (or triathlons, or whatever is challenging for you). Sometimes we just don't know what we can do until we try. And if you do try, you'll be surprised at what you can do. Another case-in-point: I ran on Sunday so I decided to go for a walk on Monday. As I headed out the door, I thought, why not run as much of it as you can and then you can walk? And you know what? I ran 2.2 miles of the 2.5 mile loop. Sweet.
I think we hang ourselves up on the fact that we can't do something, when in reality, we are smarter, faster, stronger than we think we are. All you've got to do is give it an honest shot. What are you doing to challenge yourself? Have you ever proved yourself wrong?