As for my back, well, Mondays are supposed to be cross-training days and I typically use an indoor rower for about an hour or so, but had to cut it short because my lower back seized up. I've had multiple lumbar interbody fusions and currently have ten screws, two rods, and two metal spacers in my spine. I still lift six days a week anyway, not exactly heavy and it probably never will be (certainly not if I continue focusing as much as I do on running), but you do whatever you can do.
The biggest differences getting old are it takes longer to warm up, you're more sore at the end of a session, recovery takes longer, and injuries do not heal as fast, typically lingering for months rather than days. But it doesn't mean you stop functioning. You just take your time and build up slowly. You have to learn to set goals in terms of years and decades. I worked out with resistance bands for six months before I tried to lift barbells. I walked for four years before I tried to run again. For a long time, I thought I would never be able to, but here I am.
If you saw me out on the street, I'm sure you could tell I'm in my 40s, but I don't think you'd see me as degraded or old. I've definitely got some crow's feet around my eyes and lines on my forehead, but still no gray hairs, no bald spots or receding, and the body of an underwear model. I'm not as strong as I was in my 20s when I focused on lifting and I'll probably never be as fast as I was earlier when I was a two-time state champion in high school cross-country and still ran pretty seriously in college, though non-competitively. You have to learn how to accept some level of inevitable decline, but nonetheless do as much as you can given the limitations you can't get past.