That was nice Radon. Nice and clean.

Your question about what to do with the legs is answered by asking why the figure is walking at all. How likely is it that someone who is punching (that is the main goal of the animation, I assume - punching) will step forward evenly with each punch? Try and do it yourself. It feels all wrong. There is no power in the punches and your balance is messed up because you're trying to step too quickly.

I'd suggest you try and make the animation closer to human movement, and in doing so the animation will make more visual sense - it'll be more believable. Perhaps he should take a single, larger step. Try that, punch twice and take one step as you do. Much more natural, and powerful, right? You get a more controlled (faster!) jabbing punch action, that is much more viable in a fight. The resulting animation shouldn't have the stiff leg problem and it should be easier to animate, as where the figure's limbs go will more or less decide itself.

My point is, if you're having trouble animating a movement, take a step back (hah!) from what you're working on and try to do it. It can teach you a lot about what is wrong and why and where. Sometimes you don't need to, if it's a movement you're familiar with, or if you're being cartoony, of course. It does help sometimes, though.

I made this the other day, and whilst it's cartoony in places, the way the figure throws the ball and hangs slightly after doing so came from pretending to do it for real. I admit I felt a bit silly doing it, but hey, sacrifices yano. ;)
http://darkdemon.org/user-files/16909/1352160515.gif

Hope this helps.