Our notions of what's possible affect our performance. I had a very specific experience with that this weekend. I ran a half marathon yesterday, but I couldn't use my Nike+ watch. It has an utterly ridiculous lack of ability to delete old data from the watch itself--you can only do this when it's hooked up to a computer. So, if you find yourself with a watch full of data, you literally cannot use it.
So, I had to wing it--which was made harder by the fact that all the miles weren't marked off on the course. I knew I was in the ballpark of a particular pace, but felt good and wasn't really paying that close attention to every second.
In other words, I was just running.
When I was said and done, I beat my best time by three whole minutes--meaning I was 14 seconds per mile ahead of my best pace and had shattered my previous record. My previous best had been 7:18/mi and I was frustrated in my last race because my watch was off by a few seconds, and I thought I had a 7:16 and it turned out to be a 7:20. Little did I realize the best solution was to ditch the watch entirely and instead of trying to shave a few seconds off, watching the clock the whole way, just running my ass off. I never would have even attempted to run this fast if I would have been using the watch. I didn't think I could.
Knowing what we did before, or what others did, limits our notions of what's possible. Measuring data can help you improve, but often times you don't realize how much you can do if you really dig deep unless you throw out the numbers and work hard.