The thing about lunges and stepups, (besides that they take a bit before you don't think you suck at them, of course) is that you have to be sure you're using your glutes, not just your quads. Goes much easier that way. (and I'm not saying whether or not you're doing so, just mentioning because I found at some point I stopped activating my glutes in my lunges and my knees started hurting.)
So, you'll get better.
