Have you ever been at the gym, running on a treadmill, while the guy next to you (who is in much better shape I might add) is not only running faster than you but appears to be doing it with ease. You adjust your speed trying keep up, it works for awhile and you feel good about it, but eventually fatigue sets in and you find it increasingly more difficult to stay on pace. Eventually you have to slow down to something thats "sustainable", something you can keep up for your entire workout. Learning (and working) can, and often is, just like this.