Mark, I have to disagree on ths point.
"walking will burn a greater percentage of fat than running."
Running will burn more calories than walking and burn more fat in total for the same distance travelled. As well, the higher the intensity, the longer the afterburn effect of the exercise. The percentage thing is a red herring.
Noticed this just now. That's only applicable if you can run for the same distance that you can walk, and most people can walk for a much longer time than they can run in a day, and are generally able to walk more frequently than is safe to run per week, so racking up distance is easier if you're committed to it.
Of course there is a point after which the runner has developed enough endurance and speed in his legs, that the walking guy will have to put together a freaking amount of hours to make things even. This isn't usually a problem true beginners have to deal with, they have zilch endurance and even doing 10 or 15 minutes straight is unthinkable and if they can, they're usually in shambles afterwards.
(this is for untrained individuals, people who do strength train will have much better performance right off the bat)
For overweight people there are still other options to be considered before switching to full fledged running, anyway.