Increase mileage gradually. If 30 is your normal summer starting point then increase no more than 10%/week. 40-45 is about a 40% increase in volume, that's way to much. You don't want to get hurt (stress fractures, etc.) doing too much too soon. I would tone it down a bit and start a gradual climb in volume. I've seen many high school aged athletes get fried or hurt trying to increase their mileage too fast. I've also seen some high school sprinters 400/800 type runners, run some pretty good xc times doing 25 miles/week. The main thing in training for just about anything, is don't do too much too soon and be patient.