Qi gong and short walks are a great way to start - good job!
Then you can slowly move up to a limited time of GENTLE yoga. Old people yoga. Beginner's yoga. For 10 minutes. Then in a few weeks, try 15. Then try 20.
The key is to slllooooowwwly work yourself up to more and more intense stuff (downward dogs, pushup-like poses, hopping or jumping, arm balances) over the course of several months.
Many popular kinds of yoga (vinyasa, ashtanga, iyengar, anusara, power) offer a very intense core, arm, legs and back workout... comparable to a full-on session at the gym. Way too intense for AFS energy levels without slowly working up to 'em for months.
I was doing intense power yoga but I had a minor crash (emotional Father's Day family issues, damnit!).. so I am back on EASY yoga for this week at home. Going to the easiest class tomorrow at the studio.