Riding in a 2 pt is an excellent way to strengthen legs! So is riding without stirrup. How often do you ride? the more frequently you can ride the better.
To REALLY have a good leg though it is crucial to do these exercises in the PROPER POSITION. Doing a 2 point all day in the wrong position won't do much to strengthen your leg. So first and foremost make sure your position in correct! As George Morris says, "practice doesn't make perfect, PERFECT practice makes perfect!" If you aren't sure about your position, post a few pics in the critique section, or better yet, a video.