I did a lot of walking from a couple weeks after surgery until I was down to about 100 pounds overweight. By then I was walking 6-8 miles 3 days a week, along with non-planned outings.
At about 100 pounds overweight I challenged myself to complete a C25K (Couch to 5K) program that gets you running 5k in 30 minutes in a month. Having completed this, I continued running, increasing distance until I was up to about 90 miles a week. I ultimately ran 5 marathons.
Of the two, I enjoyed running more. I did my running first thing in the mornings, when I could contemplate the day and listen to audio books. Very enjoyable stuff.
Walking and running burns about the same number of calories per mile.
I would recommend doing what you enjoy and can sustain. It's hard to sustain something you dislike.
Good luck,
Tek