I love hiking up and down hills, it's great exercise, but walking just on the flats IMO is a waste of time (unless there's cute girls around to check out). Every time I'm stuck walking without a bike I'm constantly nagging myself "why didn't you bring your F'n bike?"
What's the big deal about getting a quick-release headlamp and taillight? And stashing them in a jacket pocket?
About Machka's comment, I think weight training is way better for bone density than walking.