I know i'm late on this thread, but the best way to build your leg muscles is to ride standing. I know that alot of us can't ride as much as we'd like, but when you do, STAND UP.
When I train, I ride two or three days a week, riding as close to 60 miles as I can, and stand as much as physically possible. The first week is pure Hell, cramps, seemingly endless deep muscle soreness, but after a couple of weeks the cramps will subside(some) and the soreness won't be so bad.
I will also ride my mountain bike a couple of miles in the morning. My knees are too shot out to jog.
Bicycling is great for cardio and stamina, but riding standing conditions the muscles you use riding.