I think I was motivated to improve my ability to pedal while standing by watching the gym room at the Y during a water break on a group ride. I saw a somewhat overweight middle aged guy killing it on the stepper machine and thought, "Yeah... I can do that. It'll just take some work."
So instead of thinking in terms of the usual stuff about cycling -- speed, efficiency, conservation of energy, etc. -- I started thinking of it as a rolling stepper machine. I'd set goals for time and/or distance while standing to pedal, followed by sitting and easy pedaling to recover, then repeating until the legs were on fire.
I quit worrying about my speed on segments or average overall speed, and only focused on getting stronger. Just changing my mindset helped.
Also, I quit listening to younger coaches/trainers who never rode 5-speed or 7-speed steel road bikes with 52/42 chainrings and 13-26 "mountain" gearing. Most of the 30something or younger YouTube experts have never ridden bikes like that, so they don't understand why the old school racers did that rock 'n' roll slow cadence, using lots of body english to grunt up climbs. The young guys would emphasize sitting and spinning -- which is only possible with a 34T small chainring and/or 28T or larger rear cog. I defy anyone to ride seated up a double digit grade in a 42T chainring and 24T rear cog without rocking side to side or to and fro.
And when I revisited those classic Tour de France videos, especially with Greg LeMond, it's obvious that the bikes of that era, with the harder gearing, forced them to be in and out of the saddle constantly on rolling terrain. LeMond often switched it up, standing to stomp the pedals to maintain or regain momentum. In his prime he was my size now (I'm 5'11", 150 lbs), as were his main rivals, around 5'8"-6', 140-150 lbs, and most of them were in and out of the saddle. Nobody was telling them they were doing it wrong.
Switching between the three methods -- seated spinning 90-100 rpm in easy gears, and grinding big gears at 50-60 rpm, and standing to stomp big gears at 40-50 rpm -- did more to strengthen my legs and lungs.
I do have a lighter carbon fiber bike with easier gearing and do occasionally do exclusively seated spinning in easy gears. But with age that gets harder rather than easier. For some of us, we retain raw strength longer than respiratory capacity, so sometimes it's easier to stand and stomp the pedals, as long as the legs are properly conditioned.
I also do more walking now and some light weight lifting, using 20-40 lbs for squats and lunges. It all helps.