
Good for you for starting up with exercising and the choices that you have made.
I have been diagnosed with advanced arthritis since 1994. I have had one knee surgery and I need a total knee joint replacement on my right knee within the next couple of years. Having said that, I want you to know that I "used" to be very active. I LIKE to sweat! And, like you, I was concerned about further injuring my knees, what was left of them! Good news for us both!
Arthritis experts recommend yoga! I don't know if you knew that or not but it is not weight bearing or jarring to your joints---healthy or otherwise. If you have trouble with some of the postures there are blocks and such that can help you in modifying the positions and still reap the same benefits.
I have found that doing Pilates also is very gentle for my joints. I still have a lot of flexibility left from the years of dance classes and all the walking I used to do---just don't ask me to do squats or pick up change off the sidewalk. (I swear I would have to think if I saw a million dollars laying on the sidewalk if it meant for me to pick it up to claim it!)
I can't add to any of the rest of the comments here. They all sound good and sound to me. Abs is the other muscle group, besides walking, that you can exercise every day though. I was told that by a personal trainer decades ago. I am not sure why but you can.