You only need to work out each muscle once per week.

You should optimally be working out each muscle more frequently than that.
Check the stickied threads in the exercise forums. You can and should work out each individual more than "once per week"
If you are interested in starting to build muscle in your legs, read the stickies in that forum, then come back here with any questions.
Will walking build any muscle in the legs or thighs I know its good for your heart and a simple way to drop calories but what else