Given the arthritis, I think I would be experimenting with getting my aerobic by swimming. Without the weight bearing, flexion at the knee might be more tolerable, after a while you might get some ROM back. There are exercises you can do in the water also, you might want to check out water resistance training equipment.
I would experiment with Pilates to strengthen the core and help maintain ROM all around.
For strengthening legs, you’re probably going to have to do stuff that works your quads from your hips rather than your knees. Deadllifts may be a good move for you to go heavy with, and hopefully you can find at least one machine that will let you blast your calves.
At age 48 I weighed 270, I was mostly sedentary. At age 53 for a while I weighed 185 at about 10%. From my experience, I strongly suggest that you take it easy to start with. Go at a pace your body and motivation can tolerate. The all important thing is to build the habit of working out, and torturing yourself with DOMS doesn’t help. Just make sure you’re working to schedule: give yourself a very hard time over any missed dates in the gym. After the habit is real solid, you can start flogging it. I managed mostly without workout partners, but I don’t recommend that at all, try hard to find a workout buddy, it makes it so much more fun, and you’re less likely to skip.
Your activity level will be lower, so you will especially need diet help. See www.physiquetransformation.com. This is a computer monitored fat removal program that teaches you how to eat clean and coordinate eating and exercise. this thing was designed by bodybuilders, uses principles which Berardi is just now talking about. The hardest part of the diet is eating more food than you want to, to raise your BMR. You will learn that hunger is a danger signal (your body is shifting into economy mode). Tracking your food is pain, but the good news is with this program you can approach very low body fat levels, even with very limited aerobic work.