Weight loss/muscle gain?

I've been trying to lose weight and get into better shape so during school, I used the elliptical and at home, I'm running on the treadmill and going walking. My calves and lower legs are already toned, so I'm trying avoid things that would make them more muscular. My neighbor told me that running on the treadmill helps gain leg muscle...is this true? Will walking on the treadmill help me lose weight, but avoid gaining leg muscle?