First, keep up the great work! Walking and brisk walking are key to the recovery road.
I followed a similar path to you, with lots of walking. As the weeks passed, I challenged myself more and more with brisk walking, then brisk walking up hills. At some point I added some light running in the mix.
As far as upper body, I have an AirDyne stationary bike at home with arm levers, which allow for pushing and pulling. I started doing light work on the bike probably about week 6, being very careful to only push and pull lightly. I gradually increased how much I used my arms. Also, I used the elliptical stepper with arm levers at the gym a lot. I went light on all arm exercises until my cardiologist gave me the clearance to go 100% on all exercises, which was at my 4 month, post op check up. He actually told me that since I was past the 12 week point that I could go 100%, so I waited a little longer than I had to before I really start pushing things.
At that 4 month point, I added push ups to the mix. The bike, elliptical and push ups were the extent of my upper body work outs until about 7 weeks ago, at which point I started doing Brazilian Jiu Jitsu again. It's a grappling martial art, so it works about every muscle group in the upper body.