It soon became clear that while multiple sites extolled bike riding as a route to fitness, all the practical advice was aimed at very fit much younger people. The only thing they said to people my age was "Always consult with a doctor before starting an exercise program." The advice for younger people was also aimed at non-beginning riders. (There was, for instance, nothing about the bike skills you should have before venturing out in traffic, unless you worked your way down to teaching children to ride. Adults...should know already.)
So I read site after site, and they didn't, of course, all agree. But one that stuck in my mind talked about the three kinds of rides you should do to improve fitness. Oh good, I thought, this will be handy. One of the rides was something called the "long ride." Should be at only moderate effort, building up rhythm, muscle mass, cardiac fifness, etc. Sounded a lot like conditioning you do for performance horses if you want to keep them sound...in addition to "high impact" rides, and interval training, and gymnastics for flexibility and core strength (sounds familiar, right?) they need long, relatively slow, steady work, and they need it before you add on the other things, as well as on "light" days. So I read the rest of the article eagerly.
Then came the specifics: "Start with 25-30 miles..." WHAT? Someone's starting to ride for fitness and you think they can start with 25-30 miles? Right off the bat? At the time, I was finding one mile difficult. When I actually started, 30 yards was difficult (of course that was on a bike that did not fit me, and was of a type I had never ridden before...and on a dirt horse lot...but still.) You don't start an overweight unfit horse at 25-30 miles/day, let along a human in that state who is also over 65. However, I kept riding and riding, creeping up on 2 miles, and then 3, and so on. Someday I will reach 25-30 miles, maybe even more.
During this search I also ran across the statement that any bike riding under 10 mph was "just recreational" and hardly exercise at all...which my heart monitor proved was a false statement.
So what am I getting at here? That if you're an older adult who hasn't ridden a bike in years and decides to start again, most of the biking sites online won't give you the information you need. There may be one that will, but I haven't found it yet. Likewise, your doctor, if not a sports med doctor who has older patients, is not likely to be able to advise you on bike-specifics, and neither is a gym trainer who lacks expertise with older clients and riding outdoors for fitness. There are too many glib generalities and too little specific advice on, for instance, learning to ride a geared bike with hand brakes when the last bike you were on (the only kind you were ever on) was a balloon-tired, steel-framed bike with coaster brakes, or how to design your rides to avoid problems with aging limbs and joints. A lot of what I've done that's been successful for me is based on training horses, including dressage exercises (for instance, learning to ride a circle in both directions without wobbling...doing everything "on both reins" as riders would say, because--as with horses--just because you can ride a semi-circle with radius X to the left does not mean you can immediately ride a semi-circle with radius X to the right.
My sports medicine doc has been immense help, both in defining where a problem was, and in advising me what to do to correct it and prevent others. Almost two years of riding has improved my condition--lowered blood pressure, dropped resting heart rate, improved recovery from exercise, given me more stamina in daily life, improved my balance, etc. But it was harder to start than I'd expected (having believed too much in the "You never forget how to ride a bike" ) and it's taken longer than I hoped to get where I am now. Injuries, illness, travel, and deadlines got in the way...and should be expected. My determination to ride outdoors, part of it on natural land and part of it on...um...substandard streets...means weather is also a factor and has stopped me on days when it feels unsafe to ride.
So for people over 65 who want to ride...go for it. Just go for it with some caution, especially if your last bike was like my previous bike 45+ years ago. I wouldn't change back for anything now, but boy those first weeks were a struggle, until suddenly the coordination of gear shifting and pedaling and braking all came together. Now it's easy. Easier, anyway.