Your joints sound how a lot of mine feel .....
Oddly though, I can still ride a bike fairly comfortably.
Same here, but mines a recumbent trike rather than a bike.
I've done 6 muti-month cycle camping tours. Cannot do them any more, my bad back means I cannot get in/out of the tent without the pain in it getting a lot worse.
I used to train for them by mostly doing around 80% of expected distance, with some shorter/longer days for variety.
I had 7-8 loops around home I could go around, so never did the same ride 3 times in a row.
For more variety I had a set of slow tyre I could swap on for some ride, that way I worked harder.
I also fully loaded the trike down with the camping gear on some rides, again upping my power output on a "standard" loop.
But the most important thing is you keep it "fun", once it becomes a slog you'll back off.
As for "spinning", I learnt to do that by telling myself "I need to keep a light pedal pressure".
So no upping the pedal pressure when I got to a hill, change down instead.
After a few month of doing that, it became second nature and I finally worked out my ideal cadence was around 70-80 rpm.
I may reach 85 rpm when hyper fit, but trying to "spin" at 100 rpm is just waste of energy.
I now keep a light pedal pressure at all times now as this puts a lot less stress/stain on my knees.
So no more powering up hills, but doing this lets me cycle for longer before they start to hurt.
Luck ...
