My last surviving aunt just joined a gym at 81. She had no fun watching her brothers and sisters decline. She hired a personal trainer for 6 mos. Her grandkids joined with her.

I sit and spin at 100 in spin class when we're seated until I my legs really start to burn. Then I slow down so I don't blow up. If you think you're strong, just crank 1/2 a turn when the instructer says 1/4. That'll fix you right up.