I would appreciate some advice/tips on how others manage to stay physically fit.

I think a lot of what you, or anyone else for that matter, do is going to depend on your condition on a day to day basis. It's a good strategy to have a number of fitness activities, of varying degrees of ease/difficulty, in your catalogue of exercises so you can tailor your workouts to how you feel on a given day. Get with a rehabilitation medicine doctor to help you with remaining active within the limitations of your disease. And remember, as long as you're fighting the battle, you're not losing.

Work in progress. I bike, walk, do some low impact aerobics or step on Youtube, and occasionally swim. I stretch a LOT because it helps so much. Twice a week I lift weights. I've been doing this for years (so I worked up to it) and allow myself a lot of rest. I no longer run, can't play tennis (can't hold a racket), so I've made modifications. When I bike or lift (or drive out of town) I wear wrist braces, which help a lot. For me, moving helps so much even if it's hard to get going some days but now it's just part of my routine. FWIW, doing things like yoga or the aerobic workouts moves everything - full body - which helps (your kicking, lifting knees, arms going in all directions). Good luck, find what works for you, it's worth it.

I, too, walk on the treadmill, but due to injury to both shoulders, no weights. I do work in my flower garden and my vegetable garden, Right now, it doesn't look like it, but I am also recovering from a bout of pneumonia which has slowed me down very badly.

While I've never been one to go to the gym, I've always been physically active - sailboat racing, offshore fishing, kayaking, hiking, and there was always some project that required physical effort, like remodeling a boat, landscaping, building something. My daughter jokingly referred to me as "Hurricane Amy" because I just never stopped.
Those days of running wide open all the time are gone. I try not to focus on the things I can no longer do, but instead on the things I CAN do, and like Eric said, that varies from day to day. Creating a list of things you ARE capable of and going back and forth between them as you're able to is always a good bet.
Northernelf also hit the nail on the head both with "work in progress" (we all are), and with yoga being whole-body movement. Do a search for "yoga for arthritis" and modify the routine/poses to what you CAN do. Gentle activity is your friend now - any movement is better than no movement.
Sometimes it's as much about adjusting your mindset as anything. Finding new things you enjoy to take the place of things you can no longer do, finding new things to be excited about, new ways of keeping your body moving and your brain occupied, without beating yourself up or dwelling on the things you can no longer do. Modifying and adjusting are two things I've learned to be good at.
I went tent camping by myself three years ago, as I've done countless times, but it was a miserable trip. Unloading & setting up wiped me out, it was colder than predicted, unexpected rain didn't help, I couldn't manage the cast iron dutch oven I've used a million times with no issue, on and on. After four days of misery, I got home, unloaded, fell into bed and could barely move for a week. Then I spent a couple months thinking my camping days were done, I should probably just sell my gear, another flipping thing I can't do anymore. But when I got over my pout, I thought about what I could modify. Early spring in Georgia is unpredictable weather, so late spring/early summer, late summer/early fall is better. Do I really need a cast iron dutch oven for ONE person? Do I really need a 48-quart cooler? Sleeping on the ground, regardless of how much padding I use is not gonna happen, so I got a folding cot (after much research for one I could easily set up) and a memory-foam pad for it. And I took a hard look at my gear - that military-grade backpack that weighs five pounds empty? While it has a lot of great memories associated with it, a nylon gym bag holds my clothes just as well and is a whole lot lighter. Instead of 4-6 day trips, I go for 2-3 day trips. I skip the rocky trails and take the "easy terrain" trails and do a bit of nature photography rather than pushing my limits physically. I know the day is coming when even this will be impossible, but for now I'm learning to adjust and modify until that happens.
In many ways, this is about learning a whole new way of life and it takes time. And no small degree of patience, especially with yourself.