Martin I'd had several years of neglecting my physical health till a major crisis 2 years ago. In my mid 20s to late 30s I had used bike as chosen method of getting around and clocked up over 50,000km most of which was just within Berlin city limits. 200-300km a week were normal as I was working freelance with multiple work locations at the time.
Then came the kids, the overtime and personal neglect of my own health. Although I hardly ever rode my bike (mostly using car instead) I still maintained my membership to the ADFC (German CTC equivalent) and felt more affinity to cyclists than to motorized road users. The odd times I did get on the bike (for shorter journeys) I always felt like I ought to be able to do more and quickly pick up. But it was too easy to put of till the weather was better, or when it was less windy, or when I had a bit more time etc. But the truth was, I was no longer fit. I was wheezy and took a long time to recover if I had to run 100 yards to catch the bus- even if I could bike at a reasonable speed for 5-10km trips without noticing the strain too much. And most physical exertion left me sweaty. Another reason I put myself off commuting by bike 'till I was a bit fitter'
I got back in the saddle in August 2019 aged 49, 6 months after suffering a stroke and around 10 years after any meaningful regular cycling (or other regular sport) had took place. That would have almost certainly been avoided had I built physical activity on my daily program. Climbing the stairs, going for a lunchtime walk instead of going for coffee and cake at lunch are just little things that might have helped.
Keeping fit doesn't need to be a marathon. But to get any kind of stamina you need to keep moving.
The initial lockdown saw me housebound just as I'd started getting into a daily routine. I cannot do sport for sports sake, and although I would try to cycle with my eldest son, I was not putting the daily distance in that forcing myself to commute was doing. I dropped from 160-200km a week commuting to 25-50km lockdown km a week. Needing the kick up the backside, I persuaded my employer to tolerate me working in the office even when they were strongly encouraging home office. So from mid summer last year till now I have been one of a handful of actual staff at the office every day- the only reason to force myself to ride rain or shine and maintain the fitness I have slowly won back since my stroke. I'm not a racer, but I am rarely overtaken by cyclists, and my general fitness improvements have allowed me to substantially reduce post stroke medication as an added bonus. The bike has been the most effective medicine in my recovery.
I've managed a few 50km rides taking scenic routes on workdays usually 32km long, so I think I'd be OK for longer tours without getting especially tired. So I believe I have won my stamina back in the 18 months/9000km of commuting in all weathers. But I have also experienced how easy it is to let my health slip, and it is an uphill struggle to get back after any longer break. The German word 'Schweinehund' is not, as many believe, and insult given to others. It is used mostly to refer to one's own inner demons tempting you to give in to temptation- such as procrastination, laziness or gluttony. Your inner 'pig-dog' will tell you that the diet and fitness regime can begin tomorrow and reinforce that you have not slipped so far back that you cannot jump back in the saddle tomorrow as if nothing has changed in the prior months of neglect. And when you do take that long ride without having the fitness relying on your 'muscle memory' and brute strength, you really feel it the next day(s) and the Schweinehund will have reminded you that you'd have felt fine if you'd have listened to his tempting reasons not to get active.
Stamina is important and if you cannot ride regularly then training rides are not to be sneezed at prior to a tour and will help you build stamina up rather than throwing yourself into something your end up hating because everything hurts at the end of the day. And as you point out, from 50 onwards it is harder to maintain the fitness. I'm 51 now and know exactly what you mean. Best is not to let the fitness levels drop to begin with, then the road to recovery after surgery or illness will be far easier as any health regression will be less than someone starting with lower levels of stamina and fitness. And the Schweinehund will be easier to banish the quicker you are able to get back into healthier more active routines.