When I dislocated my L knee (hyperextension, tore ACL/PCL/LCL) playing soccer the orthopedist did his best to repair/reconstruct it but he said it would be due for total knee replacement within 10 years from abnormal wear/tear. He medically retired me from most sports that required weight-bearing. No walking for exercise; but cycling and particularly swimming were fine. That was 20 years ago. The L knee still has limited flexion and unstable side-to-side motion and is full of bone spurs, tib/fib fused together, etc; but I did very well on bone density scan and can bike without pain. It does need a long warmup at ride start to get enough flexion to pedal. I should be able to get along at least another decade before looking at joint replacement surgery.

Swimming helps a lot with overall flexibility, upper body strength, and general cardio fitness. There is a lot to be said for having an indoor sport when outdoor cycling won't be fun. Swimming with a coached masters group doing interval training is much more fun than trying to swim a mile during public lap swim and gives you social cameraderie that zoning out on a treadmill or stationary bike won't. My current strategy is to ride the bike if weather favors it, try to swim a few times a week; and if I can't get out on the bike at all, at least go to a spin class once a week.