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The most common thing doctors tell people is ‘go for a walk’. This is negligent and inadequate. I am going to dwell on one aspect of why walking, running or jogging won’t cut it. It does not give the 20-odd muscles around your knee or even the knee joint enough stimulus to remain strong and healthy. I am not counting a lot of the smaller muscles.
The muscles around the knee are ‘active’ only when you bend the knee to the greatest degree that you can do so with control. Without the bending and straightening, the muscles have little reason to remain ‘strong’. Worse yet, they fall into disuse. The joint follows.
Your knees are built to bend and straighten. They lug your weight around. They generate a terrific amount of force by bending and straightening (there’s even some subtle rotation). When coupled with the hip and ankle, they can produce even more force with movements such as squatting, lunging and jumping.
When you walk, run or jog, your knees work through only a fraction of their range. The deep bend never gets trained.
You need to systematically bend and straighten your knees as much as you can with some resistance.
What to do?
Some form of squatting, lunging and jumping. If pain stops you, find a workaround using machines, bands, hip thrusts and hinging. Remember the solution to pain is most often some movement, any movement and only in rare cases does no movement help.