I agree with Phil and Eva regarding how lonng and how often. Everyone is different, and everyone's recovery is different. Assuming the same works for all patients is laughable, in scheduling, mileage, times, whatever.
I was very fortunate with my surgery. I was up the steps with no problem when I got home, three days after surgery. I was taking five mile walks the week after I got home. The cardiologist was horrified, but he is used to dealing with patients in their late 70s to 80s. However, I never stretched my capacity, never was breathing hard, never wore myself out. It isn't a competition, and you absolutely shouldn't approach it as one. Someone who is older, has other issues, or just didn't get through the surgery so easily would ahve a very different experience.
You don't need to power walk or speed walk. You can saunter, if you like. You don't need all the little electronics: the value is much more in the motion than the speed. You don't need to keep records unless it appeals to you for some reason.
It's good to have a destination, to have places you can sit on the way (if needed - your energy may vary a lot from day to day at first), a cell phone and someone you can call (if needed), knowledge of where the bathrooms are along the way, no time limits for getting back (it may take longer than you think - having a deadline can make you upset if you just don't have good steam that day). Mostly, relax and enjoy the walk.
Best wishes,