Probably you already read this NYT article about women being too inhibited to put all their energy into sports in general (and running in particular), and therefore, having no idea how good they actually are as they never push themselves to their limits.
That's definitely true for me. Whatever sports activity I do, I never put all my energy into it. I always want to keep "a reserve", and this reserve is usually pretty big.
Today I decided to push myself a bit at running and see how fast I can run and for how long. I was surprised to see that I could easily triple the distance that I would usually run, and moreover, I would do it in less than twice the amount of time that I would normally take for a third of distance. The most surprising thing was that I still did not reach my limits. I did not find out where they are as I had other appointment that prevented me from continuing my little running test.