I had a lot of knee pain, cpm. I had it behind my knees while seated, and had to have my legs elevated so that all weight was off my legs and only on buttocks. That lasted in my case for about nine months, then just stopped. As my walking grew worse, my feet sort of slid forward in a sideways position so I wouldn't trip over the "foot drop-toes". My knees suffered then and became swollen.
When I began using a power wheelchair, the knee abuse stopped and while seated I was fine. However, my knees hyperextend as a result of muscle loss, meaning when I lie flat, my knees dip down toward the bed. I have to have a pillow under them to hold them up. And now... only tendons surround the knees, my feet in bed want to drop off to the side and I have my husband shove soft fabric along the knee on the side to keep the leg straight, and also make the feet (toes) straight up against a firm surface. Otherwise, pain awakens me in the night.
I think that you really need to begin the process of getting a power chair now. Do contact your ALSA coordinator for finding a loan closet chair. I've read from others that when these joint problems aren't taken care of, they become more serious. I hope you've already found your local ALSA and MDA, as they both can help you greatly with the equipment you will need.
Best wishes... and my sympathy for those knees.
Ann