As a knee suffering athlete I’d strongly recommend going for a spinner bike or other stationary bike over a treadmill.
Pounding our joints (oo-er) is what causes our knees to be goosed in the first place.

The bike is pretty much zero impact Tony and will still give you the cardio you’re looking for.

As an aside, I’ve done fasting with exercise at my age (I think you and I are similar in age, 62 here) and it REALLY strips fat off. Fasting = lowers sugar = body will burn fat instead of using sugar.