Fallout from Knee Surgery

For many years, I took a hard step aerobics/ kickboxing class. I even took a kettlebells class. Exercise became very important to me since I have a chronic health condition and my doctor told me, and I quote, "You need to get off your dead ass and exercise". Really, he said that to me.
So, I listened. I took this class for at least 12 years, 3 times weekly. I slimmed down, got healthy and felt good. When the instructor got pregnant, I even taught the class. I was good at it.
More than a year ago, I sat down on top of my step box prior to class starting, and felt something in my right knee go "sproingggggg!". I literally heard it and felt it. I was shocked. When class started and my right leg stepped up on the box, the pain was intense. Yep, I musta broke or snapped something.
I left class. Took the next 2 classes off. Went back. Again, too painful to step up on the box.
I went down to the smallest box, what I called "the weenie box". I could do the exercises but it just wasn't the same. And it was still painful for me, just less painful.
I had to give up the class. I had been a part of that class for so long, that if I wasn't there, other people stopped coming and the class disappeared. And I'm not saying I'm Miss Personality, what I'm saying is that many of the younger ladies saw how strong and slim I was at my age and were inspired. And they kept coming in order to become strong like me.
Back to the story.
I gave in and had knee surgery in October. The doctor got in there, said there was more osteoarthritic damage than he'd expected, more than he'd seen on the MRI, so he cleaned up a bit, and just closed me up. He said, "You're looking at knee replacement down the road."
So I have tried walking on my lunch break, doing situps and leg lifts at home, and I'm losing my battle.
I can just fit into my work clothes now, and I'm uncomfortable in them. I've gained about 9 lbs. I'm short so every lb. counts on my frame.
What do I do now?