Interesting comments.
Yes, starting off healthy is a huge advantage.
I still recommend running. Mainly because you are outside covering ground, and if you're walking 3.8 miles, who knows you might be able to run a half mile. At the end, you'll have pumped up your muscles with blood, and accelerated all your bodily functions like heart, lungs, blood flow.
Unless walking makes you breath hard, I think you should speed up.
I wouldn't run more than a 2 miles at a time. Running is too rough on the body (knees, feet). Find something else if you want more exercise.
You'll do more good and it won't take so long.
Of course, my opinion, but I feel like it worked for me. A while back I was just walking because my feet hurt. Graduated to running, got back into lifting, currently on a 3 year swimming binge.