How much exercise should we do to keep fit?

A. Before we became organised into farming and eventually urban communities, our ancestors walked and ran around 25 miles a day. Once they had caught their food they ate until they were full, then slept and rested until we were hungry again, maybe two or three days later. We haven’t changed physically since then, but our lifestyles obviously have. The consensus is that as long as we don’t eat until we are satiated at every meal we can keep fit by exercising briskly for an hour a day at least four times a week. New research has even suggested that if we exercise very strenuously (sprinting at our highest speed) for only a few minutes a day, we could achieve the same effect. I’d like to see that repeated before I would advise people to do it.