Hi Helen--I am a bilat also and, for me, pre-surgery exercise was the single most important thing that I did to assist in my recovery.
I don't know your level of fitness or of fitness knowledge, or whether you currently belong to a gym. I had been a fitness junkie, but had not done anything for several years because of my limitations. Six weeks prior to surgery, I got a prescription for Celebrex, which allowed me to use the local gym without too much pain. Not sure if you will need a prescription, and you also have much more time to get in better shape than I had.
I did a relatively simple, straightforward weightlifting regimen, using light weight and all using machines, including leg press, leg extensions, leg curls, bench press, lat pull downs, shoulder press, bicep curls, tricep extensions, and sit-ups. Leg raises, without weight also were helpful and you will certainly appreciate the time you put in afterward.
I think the best exercises for me were the leg extensions and leg curls because they allowed me to feel my muscles, to move my legs post-surgery, and to recognize when and how I was improving. Also, you will be using your arms more initially, so the tricep strengthening exercises will also help a great deal.
If you can join a local gym, then this will help. If not, then swimming will definitely help a great deal as it is very low impact and works everything. I am swimming regularly now to assist in my recovery.
Congratulations on your decision. It was one of the best decisions I ever made.