I also have a reconstructed ACL, with a follow-up arthoscopic surgery to remove torn meniscus. Actually, I think HAD is the better word. LAst time I went to a dr he said the reconstructed ligament was tearing.
Anyway, I'd been going to pilates for months. We built up, gently, a lot of strength in my hamstrings (esp on the bad knee side). I was feeling good and strong so I started doing a program that included squats and interval training. My knee's verdict: WRONG!!! I wasn't even using weights but it KILLED my knee. It's all wobbly and I feel just like I did before my surgery.
Anyway, I'm rediscovering an old exercise I used a few years ago called callanetics. It's super super gentle on your knees and back. And it totally works. It's not the most exciting, but it does work and it is gentle. Pilates is another option. It was designed for soldiers who'd been injured in world war I--joseph pilates made up a way for the men to exercise from their beds despite their injuries.
You CAN work out safely and effectively with a bad knee. In addition to walking, you will want to do some strength training for that injured leg
(whether it be pilates or something else non-weight baring). If you don't, trust me, that bad knee side will lose more and more muscle and the leg will become weaker.
Then, your body will be thrown out of whack bc your other side will start to overcompensate and you WILL most definitely start to experience hip, back and shoulder issues. BEEN THERE, DONE THAT.
The thing no one tells you is it's not just about the post-op rehab--it's a lifelong struggle to keep that injured leg as strong as your non-injured leg. Still now, after months of working on this issue, my injured leg is smaller than my non-injured leg. As you have discovered, it's VERY hard to keep the muscle where it needs to be because you can't help but favor that leg.
Talk to a physical therapist about some safe exercises to keep the strength on your "bad" side. If that's not an option, there are many website, especially ones for runners, that talk about safe non weight-baring ways to gently build strength in your hamstring/quads on your injured side.
Here's one great low-impact way to build strength on your injured leg. (A pilates instructor who helps rehab runners with knee injuries showed this to me.) http://www.youtube.com/watch?v=lIVhLWZba5o
Try with two legs, but you'll want to use one leg (if you can, if not--build strength first and then focus on injured leg) to get that muscle back. You can do it. This IS fixable if you put in the work.
Whether you think you can, or you think you can't--you're right.