The issue of exercising while in afib, I think, depends on the type and severity of the symptoms experienced. Some people feel weak, tired and dizzy. So exercise is often not possible until normal rhythm is restored or meds (BBs) are taken to lower the heart rate. On the other hand, there are a lot of folks who don't experience any symptoms at all while in afib (some don't even notice they are having an episode). I'm one of those and I have no trouble jogging and exercising while in afib (without the atrial kick I tend to get winded a little sooner but not significantly). I know an 80 year old gent who is in continuous afib and goes to the gym daily for a treadmill and weight work-out. It's true that some of the meds tend to limit intensity of workouts. In those cases low intensity, long duration workouts are preferred (even long hikes).
Now a bad knee is another issue entirely. In my wife's case, osteoarthritis has damged one of her knees badly. She has scheduled a total knee replacement in the fall just so she can keep up with me and the grandkids (she has already had arthroscopic surgery).
If you can walk without a limp, long slow walks will burn calories almost as well as jogging over the same istance (just takes longer) and so helps weight control. There's also swimming and bicycling.
Thanks for your comments. I am one that does get weak during more strenous workouts. Riding a bicycle will even get me weak. Since I was always very active and could really push my workouts, it is frustrating to see how little I can workout today. Will my endurance begin to grow if I stay with taking the long walks so I cycle again? Do you suggest weight training?