Speaking to the OP's question, I started riding again at 50. I joined my group at about 55, so that was 5 years of gradually increasing weekly volume. I'd ridden one flat, solo century before that double metric, which was somewhat hilly and all the riders were stronger than I. The usual metric is that your longest day ride can be as long as your weekly mileage for the last few weeks if you rest a couple days before. So the way to increase your distance is to ride more. I rode a 154 mile, 9,000' event ride last summer at 77 y.o. The oldest rider to finish that ride that I know of was 82. Back in my late 50's and 60's, I usually had about 5000 annual miles. 100 miles a week will get you quite fit. I never averaged more than about 120 miles a week in the summer, but my one key ride was about 60 miles and 3000', ridden hard enough that I could hardly walk at the end and usually had to lay the bike down and step off of it to avoid cramping. For training purposes, a 4 hour, hilly long ride, ridden hard, is about right, never over 6 hours. Going longer mostly means you go easier to be able to do the distance and that's not as good training as going harder for 4 hours.
Anyway, I never decreased my longest distance, though I had to decrease my weekly mileage because it became harder to recover. It's sort of self-limiting, probably different for everyone. I didn't get much slower on that 154 mile event, going from about 9-1/2 hours to 10 hours over ~20 years. I once took a winter off the bike. It took me the whole summer to get it back and I never did that again. Consistency is the key. "Never give up." Some people say, "Never stop," which is about the same thing. It's kinda the only thing you need to know.
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