Another non-runner here. I ran obsessively in my 20s and destroyed my feet--I'm very lucky I can walk for exercise, because that was off the table for years. What works for me best--because it's one thing I know I can do--is to wear a pedometer, set a step goal, and make sure I move throughout the day. Maybe one walk expressly for exercise, but otherwise adding walking to my day wherever I can.
That said, if I could run, I would, because the endorphin rush was like nothing else I've tried.