Question abut girls and muscle?

So I'm a 16 y/o girl. My body fat is really low so any muscle on me is really visible. Unfortunately I build muscle REALLYeasily. I was happy with my thin legs and flat tummy(I did a few crunches and such, but nothing to make bulging muscles, just flat and toned). However now I'm starting a winter training program for track in the spring, both of which include alot of weightlifting!! It's only been a week and I can already see the muscles formin in my stomach and legs! I don't really mind seeing muscle in my legs, but they are LARGER, as is my tummy, in clothes and such, and I HATE it. I want to be slim, not built and athletic looking! How can I find a middle ground that won't hinder my track performance??advice?