All origins of man aside, we basically have hunter-gatherer genetics. I view our needs for exercise with this in mind. When it comes to aerobic activity, I would challenge anyone to come up with a predator we can outrun for 45 minutes. On the flip side, what prey would we pursue for 45 minutes? The answer, of course, is none for both questions.
Envision instead, your ancestor bursting out of the brush to through a spear and chase down a wounded animal for 100 yards. Sounds more reasonable. For this reason I think we are better designed for burst-type activities, and several researchers studies over the past few years continue to support this notion.
This means that walking, as a form of exercise, is not even close to what our bodies need. I always hate to discourage those starting to exercise, but the fact remains that merely walking does not really have all that great of a response in our bodies. The good news is that intermittent short burst aerobic exercise is very easy to adopt and can take literally only minutes per day.