Part of the reason recovery can take so long has to do with the knee joint itself. It is a very complex joint with a symphony of muscles, ligaments and tendons involved in its mechanics. You spend many years with arthritis slowing changing the dynamics of the joint and your ability to move correctly. Gradually, over time, your body adjusts all these soft tissues to compensate for the joint degeneration.
Then along comes joint replacement surgery which immediately puts your knee joint and your body back in proper alignment. Bones and tissue mend where they were cut, reamed or manipulated. But for those soft tissues that had realigned themselves, the process to readjust is not so quick. This is the part that can take so long. You must allow your body to realign and then strengthen those muscles that may have been underused during those years you were out of alignment. During this adjustment time, you can have pain and even swelling if you push a bit too hard.
It certainly can be a major recovery. Not everyone experiences recovery in the same way because not everyone's degeneration occurs in the same way. The best advice I can offer is to pay attention to what your body is telling you. You'll know when you're pushing it. Back off a bit and then start again. Slow and steady progess is better than hitting it hard and trying to regain strength and flexibility all at once.