The great thing about cycling is that it's low impact so it's a good way to get back into exercise. If you can't walk well, aging is going to be even more unpleasant than it has to be so I'd recommend working on strengthening your body so that it can handle walking again. Cycling is far better than not exercising at all, but I would recommend walking to anyone over cycling if possible.

For many years, I cycled > 10 hours per week. But these days I only walk and haven't ridden a bike in over a year. Even though cycling can be a lot of fun, I don't really miss it. When walking you have a different relationship with the environment. You feel like a part of it. Cycling is much closer to driving than walking. You are just passing through. It's also far more dangerous so you have to focus more on the road which allows your mind to wander less than you can while walking. There are things that you notice while walking that you would never see on a bike. I once watched an epic battle between a caterpillar and a wasp. You realize there is so much more going on in the world than what we tend to focus on.