I'm doing a lot of trail running and some XC races as my primary means of ski fitness this off-season, but my mileage is about double yours and I'm planning to do a lot of high-intensity hill repeats in the coming weeks to condition my quads and lungs for the slopes. If you want to get set for ski season, you're really going to need some more challenging workouts, like tief said.
Sounds, by the way, like you have the same kind of clueless XC coach as I had in high school. Try to do some harder stuff on your own. And, next season, start building an aerobic base in June with primarily easy runs and work your way up to 40-mile+ weeks by the time school starts. You'll be running 6 minute miles in no time.