Researchers at the University of Missouri - Columbia believe that what takes place during the rest of our waking hours is just as important as structured exercise, if not more so. Lead reseracher (Marc Hamilton) discovered evidence that sedentary living promotes decreased metabolic activity and can stimulate the disease process despite daily hourlong exercise programs. (online study - 9/7/07 - doi: 10.2337/db07-0882)
During our 2 million or so years of evolution into homo sapiens and most of the time since 2 million years ago we didn't spend much time sitting around, so its not surprising that the sedentary lifestyles we have now for most of our waking hours are not good for us. A burst of activity for 30-60 minutes once a day is not the same as moving around all day, looking for roots and picking berries.