I'm with Reakt on this.
Sustaining a 9mph pace for 21 minutes shows you have a decent aerobic capacity. It also means that your recovery won't be hidiously long when you start sprint training, so you can start off on a good stead (I mean, whats the point of sprinting for 30 seconds if you need 2 minutes to recover, from a conditioning pov, you'd be wasting your time)