Many of the contributions to this thread seem to be saying things which are very similar, but from an individual's perspective. I think I'm about to do the same thing.
I'm still 55, but only just. I've always ridden much the same way - alone, reasonably briskly but always at a comfortable level. Compared to myself a quarter of a century ago, I'm about 2-3 mph slower than I was. It's very obvious that I'm not able to increase the effort on the hills as I used to, so I drop down and take it slowly. But echoing what many have said, the distance seems to be coming more easily. Overall I'm happy with this progression and hope I can continue in the same vein for a good few years.
About this time last year I started to enjoy cycling again having done very little for a decade or more. I made the step up to 50 miles fairly easily, and after about half a dozen rides of that length I unexpectedly found the confidence to go up to 100 miles. This year I went straight back to 100 miles after about 3 months off.
But I'd be fooling myself if I suggested that I am or have ever been particularly fit. I have a method that works for me; once I have the muscle tone, at my pace 100 miles is pretty easy. Many other things I do, which most people would regard as normal activities, I find more fatiguing.
For various reasons I've been off the bike for a couple of weeks. I expect to go out on Tuesday when my intention will be to do 150 miles, but I won't commit myself to that distance until I have a good idea how I'm feeling. It will be interesting to see how that goes! If I'm successful I will break my record for the longest period off the bike prior to a 150+ ride, but I did set the existing mark for that earlier this year.
It's a voyage of discovery.