With something as important as longterm health (weight, cholesterol, bloodpressure control), and with your limitations on the various gym machines, seems like swimming would be an ideal fill-in. To lessen the time you need walking, treadmilling, elliptical, and all the other gym "machines".
I know you said you'd rather do the stuff at home, but this is your longterm health you want to maximize---and that includes how you control your weight without prematurely wearing out the knee fixes. Swimming seems ideal fo doing both. It will take a decision to spend the time going to the swimming facility, but if you don't maybe you'll spend that time going thru more knee fixes? Besides, a routine going to the swimming facility 3 or 4 days a week could become a pleasant routine to look forward to. Get you out of the house, seeing new people. Could be a good thing.
I belong to 24 Hour Fitness where I live and they have a swimming pool. They have locations around the country. Maybe there is one near you. I waited till they had a special enrollment deal, no initiation fee, a one-time small processing fee, and if you paid up 3 years at the start, then renewals for your LIFETIME are at $20 a year! (They can do this because the average new member exercises for SIX WEEKS and quits, never to return!)
Or you can get the special deal with no enrollment, and pay monthly---I think it's about $24 a month. Lets you in 4 days a week (youchoose) use of all machines, pool, sauna, steamroom---basically the run of the place.
For those who travel a lot, they have a higher priced dues schedule lets you visit nationwide locations and get in 7 days a week (who would do that!)
Anyway, from what you say, swimming seems a good solution to multiple problems.
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