The short answer - build up slowly. Do some things apart from cycling, add stretching or yoga or some such thing to what you do, and again, take it easy and slowly work up. A little bit done daily with a gradual increase really does lead to a great deal later on.

Not necessarily a training plan per se, but - I am 59. I had cycled in adolescence and early adulthood, quit riding in my early 20s, smoked a LOT of cigarettes, got back into bikes at 37 while still smoking, built up to 2.5k mile years and had a massive heart attack at 42 followed by quad bypass procedure. Quit smoking obviously, did cardiac rehab and when finally allowed to got back on the bike. Through the fall, winter and spring of that first year I rode lots, slowly, on long slow rides with a patient friend. We rode the back of beyond stuff, bumpy old tarmacadam far from the wannabe racers, we took it easy, and it worked. I got back up to almost 4k miles the next year, nearly 4.4k miles the year after that. My speed came up, I lost weight and felt pretty good, I took up mountain biking, then single-speed mountain biking - and then we adopted and had a second child in the same year and cycling pretty much ground to a halt. I puttered around and did a few sporadic miles, and I commuted by bike, but that ended when we moved to a less accessible neighborhood. I will note that during the years I commuted, which was a whopping 3 miles a day in .75-mile increments, coupled with climbing stairs, I still had some muscle tone and could still do the occasional weekend ride.

When we moved five years ago to a neighborhood further out that did not appear to have a safe cycling route to work, coupled with family schedule changes, I stopped cycle-commuting and my condition nose-dived. Then came 2020, lockdown, etc. I really don't know why I sat on my ever-wideing a$$ so long, but I woke up one morning in June well before dawn fed up with the pandemic and my own passivity. I hung lights on my favorite beater Gitane fixed-gear and started riding before dawn on the essentially closed-course 1-mile loop that is my cul-de-sac neighborhood. At first it was 3-5 miles, but i would remind myself that 3-5 miles beats the hell out of zero miles and I had to start somewhere. I got on nodding and waving basis with a couple of dog-walkers, a runner and the occasional fox heading back into the woods and looked for things to see or think about so that I wouldn't equate what I was doing with a hamster on a wheel, and it worked. I came to view the morning laps like a sonnet or a quatrain or haiku, where the limits ARE the point, sort of.

I found that just riding 30-40 minutes per morning this way restarted my metabolism. I dropped about 15 pounds just getting up, riding for a little while, then showering, eating breakfast and starting my day. On weekends while my family slept, I started going out at first light and riding for an hour and a half to two hours. I started riding a relatively flat out and back until I felt ready for hills again. I focused solely on having a nice ride, using the bike computer for a time piece and a navigational aid and not focusing on speed. I made sure I was willing to stop and photograph stuff, or that I would pause long enough to really take in rays of sunlight coming in through the leaf canopy over a narrow road, or fog around a '30s Ford truck with a for sale sign, or the gentle curve of Dixie Drive. I dusted off my other bikes, but primarily rode fixed-gears so I could practice different cadences and use every position I had on the bike to work every muscle in my body. As I got stronger and could ride further and still not be wiped out when I got home, I added dirt roads, then began challenging myself by taking a route that includes the steepest, nastiest, mercifully relatively short climb in the county and grunted my way up it. When autumn came I kept doing it, riding before dawn with lights on a relatively safe nearly closed course during the week, just building it all back up.

I was riding 25-28 mile rides consistently on the weekends, typically 10-11 miles each morning before work. Based on that, I would feel confident I could ride a half-century and probably a metric century if I paced myself. I rode more miles this year than in any year since 2008, and had I started seriously riding in March or April instead of June, I would probably have a nice number for the year - but I'll gladly take 1,100-odd miles over zero miles.

Right now scheduling is challenging for a plethora of reasons I won't go into, so I have learned that on days I don't ride I can get up and spend 20-40 minutes stretching and doing basic exercises based loosely on the old Canadian Air Force 5BX stuff. My parents both passed about a year apart not too long ago, and the examples they gave of what limited mobility looks like spur me to do things to train to maintain balance, flexibility and strength starting now.

Good luck and keep us posted!