Though never an exercise for exercise sake type, i did work super extra hard after the knee op for a couple of months. Then on advice eased. Now more walking, and standing up out of chair without arms. And lucky i have some 15 high steps in the house. The physio woman said they are a good "strength builder" too.
The pain was so bad in the beginning no way i was gonna let that damage last. They say the post knee op exercises are the most painful and toughest of all the ops, as hip and shoulder. The first about 2.5 months i hit very hard. Doubled up on the exercises given me, like this stuff .. https://orthoinfo.aaos.org/en/recovery/total-knee-replacement-exercise-guide/ . As said earlier, three hours a day.
Being told the muscles et al were tangled up after the op. and that the idea was to get them straightened to natural positions as quickly as possible was the key incentivizing information. After about a month the flex exercise still hurt the most. I got it to just over 120 degrees .. https://www.onestep.co/resources-blog/range-of-motion-after-knee-replacement-surgery .
A guy in the hospital who just had a hip done told me he had a friend who had his knee done. He said his friend only made a minimal effort at the exercises and two years later was still suffering discomfort. That was incentivizing for me too. ;-)
I give all this only in the hope it might help someone out there.
Your "I cannot walk for aerobic benefit." sounds lousy. Hope that just means that you can still walk but just not fast and hard enough to get the good aerobic effect. Hope that points to the knee again which, if, could mean yours is as bad as mine was. Hope your lungs are not so bad you can't walk. Think maybe you have said you still play golf. Hope so.
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