I'm in favor of walking.
Personally, I don't think our bodies are meant to run long distances. To me it seems more like we are designed to walk long distances and run/sprint for short distances to avoid danger.
Running is hard on the body, it puts a lot of stress on the joints. As we get older that starts to take a greater toll. I may be a bit biased having already had some joint problems, but I feel that running isn't an ideal exercise. Although, it is possible that with future advances in regenerate medicine, we will be able to regrow cartilage and that will become a non-issue.