It's hard to get used to being considered "older" at 51, but ok, I'll participate.
To supplement my riding, I walk a bit, hike a bit, and lift weights at the gym consistently.
My lifting aims to hit all major muscle groups (but not on the same day). If I don't work out my shoulders I get shoulder pain. If I don't work out my back, my posture suffers. If I don't work out my legs and core muscle groups I get bad at crouching down to grab pots and pans from deep inside a cabinet below the kitchen counter. ...and so on.
If I do work out all these groups, I find myself better able to ignore the fact that I'm 51 as I go about my daily life. There are many studies that suggest aging is a better experience for those who retain muscle strength, particularly in their legs and core. Before I started lifting I thought my legs were pretty strong for all the biking I do. Then my brother taught me how to squat, and I discovered just how tremendously weak I really was. I was concerned that even with light weights on the Smith machine I would squat down and not be able to get back up again under load.
That was six months ago. Now I'm squatting more reasonable weights, doing dead lifts and roman dead lifts with body weight or more, hack squats, and many other lower-body things that I didn't think I could do before. So weight bearing exercise is highly useful as we age, and not just natural exercise such as walking, but also exercise that forces us to do things that we tend to avoid as we age; squatting, lifting with our shoulders, lifting with our backs (safely), etc.
I'm continuing to improve, and as I do improve cycling is also getting easier. But even more importantly, little aches and pains that I had almost put out of my mind because they had been with me for so long are actually going away entirely. The price for this: $10/month, about four hours a week in the gym, and nearly continual mild muscle aches from 'DOMS' (delayed onset muscle soreness, from working out).