Only 75 here. I do a long event ride most years, hoping to do it again this year, but maybe not without vaccines. It's about 150 miles and10,000' of climbing, a long day ride. The last year I rode it was 2019, I was 74, and I averaged 14.9, moving time. That's somewhere near as good as it gets for ordinary folks. Every year, the oldest finisher is about 80 and they sometimes have trouble coming in before the course closes, so a 15 hour ride. Riders slower than that are swept off the course.
But back to reality. The big problem for an older rider just starting out it this age limitation business. Every year you're weaker, so that really sucks. On the other side, if you train smart, every year you're stronger, so there are a few years where you come out even or maybe even gets stronger. Training aerobic ability is very slow, takes years.
So that's the good and the bad news. IME the best way to start out is, as mentioned above, to increase the length of one ride a week, making the rest of them say 1 hour. As also mentioned, consistency is key. I usually only take 1 day/week off, though some of my rides are fairly easy. When I was starting to increase my distance, in my early 50s, I'd ride away from home until I was tired, then ride back. The other thing I did was to ride up every hill I could find, not on my long ride day though. Cycling is all about riding up hills. I used a heart rate monitor from the very beginning. And you should have your doctor's permission to do all this. Most doctors encourage this sort of thing, but not necessarily for everyone.
As we age, we need more strength training and protein to match. Sarcopenia is a geezer's enemy. I don't recommend going to a gym this year, thst's for sure, but there are inexpensive dumbbells on Amazon.
https://www.amazon.com/AmazonBasics-...dp/B071WSFSGC/
I bought 2 sets to get enough weight, still inexpensive for what they are.
There's a 65-85+ thread at the top of this sub-forum page.