Have you considered switching from a threadmill to an exercise bike? That way you'd still be doing cardio, but putting far less pressure on your joints and being in less of a risk of injury from the high impact of running?
I found cycling has also been really good for my knee pains, as the muscles supporting the knees have got stronger. I also do pilates, which is so great for my overall pain levels and fitness, and zumba (although I can't do all moves and I have to do everything within the limits of "normal" movement, rather than my wider range, but that just makes me work harder so it's fine).
I've cut down on a lot of junk food as well as doing more exercise and have definitely lost weight. But I don't quite dare to get on the scales to see how much is gone, mainly because I worry that despite my clothers having got a bit too big for me the scales won't show any change. Silly really, but there we are.