I do kick-boxing and it works each and every leg and buttock muscle. It takes just a month to see a tremendous difference.
Yoga's great for relaxation giving you flexibility, which is helpful for doing other kinds of activities, including just day-to-day activities. It requires some muscular strength, so I wouldn't stop doing it, especially if you like it.
But try starting some cardio and work your way up to more intense workouts. Then hit the weight room. Squats especially will be effective in building leg strength. I do them frequently throughout my day, just get up from my desk and do a bunch of squats, or I'll run the stairs in my building at lunch time when I take breaks from the computer.