Kaitlyn,
You probably don't need to increase your mileage too much, but if you wanted to you could gradually work up to more of 45-50 mpw. Only increase by 10% maximum each week though, the last thing you want to do is get injured. If you don't want to increase mileage you can always add pool or bike sessions for some added aerobic fitness without the impact on your legs.
I wouldn't worry about weight too much. It will come when you're doing the work (assuming your diet's not horrible), but I raced college cross maybe 5 lbs lighter than you are right now and am a half an inch shorter than you (granted I carry a bunch of muscle, so you are definitely the expert of your body) and was in some of the best shape of my life. While losing weight might make you faster, you just want to be really careful about eating disorders and such. They can destroy your career over time if you're not careful.
Some good workouts for building strength that we'd do over the summer would be fartleks for like 2-3-4-5-4-3-2 min with equal rest in high school. I also found hill repeats to be really helpful (we'd map out ~1000m loop with probably a 250-300m hill and go up the hill hard then have some alternating sequences of rest and hard around that loop) for building strength and attacking those cross courses. Tempos are pretty tough but also good.
I agree with the other posters that it would be helpful to know your pacing. I found that my times radically improved when I really tried to be more aggressive during the 2nd mile of the 5k. I also agree with them that you shouldn't worry about going fast on easier days. Listen to your body--you don't want to be burned out before your racing even happens.
Best of luck!