The only way to get better at running is to run as much as you can. If you're doing xc, I'm also assuming you'll do mid or long distance in track (800m-3200m). If your school's xc/track program works the way my school does, you can just join in by asking the head coach if you can join in as a beginner to work up to cross country season next fall. They will probably tell you the amount of miles and weight room days that'll fit you best. Just go along with it and finish as much as you physically can. After like 3 weeks, if you've pushed yourself everyday, you'll feel ready to tackle up to 4 miles, if you aren't running already.
Are you concerned about the amount of work for muscles or the physical look of muscle bulk?
Regarding the work: Muscle/tonedness will come naturally as you keep working. Distance runners usually don't look as bulky as sprinters, or do as much weight lifting, since they utilize "slow-twitch muscle", which are long and lean. "Fast twitch muscles", which sprinters use for explosive speed, are shorter and bulkier.
Regarding the aesthetics: Look up the Nike Cross Nationals girls races. Top hs distance runners, and they don't look bulky at all!
And to do well and not get injured: Eat enough calories (Don't be afraid to pig out, but of course, not on junk food or to the point of throwing up), push yourself (while using strategy) until your legs go weak, slow down or relax when you begin feeling any pulling pain in your muscles, and stop completely when any sharp pain arises.
Also:
1) isn't it track season right now?
2) what hs class are you, and is this your first year ever doing xc/track?
3) if you want any more details on cross country/track, I got you! I love this stuff and I started out going from a complete beginner to jv1/varsity over one summer! This only works if you are serious about the sport and have time to practice extra by yourself over the weekends!
