I have arthritis secondary to old jump injuries, and dramatically reduced range of motion in my neck, shoulders, and back. I have service-connected VA disability ratings for all of them. Then, we get to knees and ankles. Same story.
Here is what is working for me.
1. I lost a ton weight through strictly controlling my diet. In my case, I eat only Thai food. Lots of green vegetables (colloquially known as "rabbit food"), and lots of fruit (some of which has no known English name, and looks like it is either the result of a nuclear test gone amok or teleportation from the era of the dinosaurs). I cut down on meat, almost eliminated milk, and completely eliminated breads.
2. I got my ass into the pool. I crank laps for an hour every day. It took me time to work up to this. If you do not know how to swim well, for therapeutic purposes, consult a physical therapist or a trainer, and learn. Pool exercise is the shit. It is easy on the bones and joints. My back, neck, knees and shoulders are still fucked up, but I feel a lot better, and my mobility has dramatically improved.
3. I started taking testosterone in gel-cap form. This really helped me grunt through the pain and discomfort barriers as I was improving my physical conditioning. It also facilitated the conversion of fat back into muscle. It also made me a lot more mobile.
4. I married a woman twenty years younger than myself.
Got to go. Heading to the pool now.
Good luck.