I'm a few years ahead of you, age-wise. I hadn't done much exercise-wise after high school, but in 2010 I started doing walking as a time-killer while waiting to pick up a family member after work. That kind of mushroomed into doing a 5K turkey trot the following year and an 8K in early 2012. By then I had expanded the walking to around 3 hours on Saturday mornings. I figured that doing a half marathon wasn't that much farther than I was already doing, so I tried a half in San Jose the following September, and finished well within the 4 hour time limit. In 2013 I did the 8K again, a half in SF, the half in San Jose again, and a 10K turkey trot. It took off from there, doing 10 halfs in 2014 and 13 halfs in 2015 (plus events of other distances.
I slacked off a little in 2016, a bit more in 2017 (fewer halfs, more 10Ks), and since NYE 2017 I haven't been able to complete a half, focusing instead on 10Ks. Sort of in the background, it was discovered that I had heart rhythm problems - very slow at rest, and sometimes skipping beats while sleeping. I didn't notice it particularly, and thought it didn't affect things I was doing. My cardiologist had me on a monitor and saw this it was slowly becoming more pronounced, so with 2020 and Covid also came a surgery to take care of it.
I was still being a bit lazy, and then I had Covid. As part of completing the recovery I picked up my activity level, and when in-person timed events resumed I was surprised by how well I was doing. A week and a half ago, doing my second 10K since Covid shutdowns, I had a finish time better than I had been able to do in 3 years. I've registered for two half marathons to see if I can again do that distance. My current guess is that my heart rhythm problem was affecting my endurance, by not responding properly to increased exertion.
Part of picking up my activity level, post-Covid, has also doing some work with weight machines (at the gym) and hand weights (at home). I'm trying to achieve balance in whatever degree of fitness I reach.