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.

Unfortunately, you possible just need to walk more then and focus on your core muscles. I have the exact same thing and the only way to stop my back from aching is to focus on tightening my abs. Doing some basic ab and core exercises like planks, superman, etc will probably help. You need to do them correctly though and rally focus on tightening the right muscles, otherwise it'll hurt the back even more.

If you have the means, I'd probably recommend visiting a physical therapist, or a personal trainer who specializes in core strengthening.

if walking hurts your back then you probably need to lose belly fat. Even though I am thin (6'4", 190 pounds) I have found that with an ideal diet (mostly red meat) and some moderate exercise I can lose 10 pounds of belly fat and my back pain drops off a cliff. If I get down to 170 pounds then I have almost no back pain. At 160 pounds I feel like an elevated being.

Walking shouldn't hurt your back and its very worrying that it does, you should look into some resistance exercise.