Was the course completely flat? A lot of cross country courses are purposely very hilly. If the course is hilly then you need to train on a hilly course as well.
To improve your time you need to run at least double that distance once a week, and varying shorter distances on other days. If you only run something like this:
M - 3 miles
T - 2 miles
W - 3 miles
Th - 2 miles
Fri - 6 miles
Sat - rest
Sun - rest
Or if you also strength train:
M - 3 miles
T - weight training
W - 3 miles
Th - weight training
Fri - 6 miles
Sat - weight training