For some time, scientists have been intrigued by the idea that breaking up exercise into repeated, short sessions might be as beneficial as longer workouts, and most related experiments to date have been encouraging. In a 2012 study of people with symptoms of hypertension, for instance, volunteers controlled their blood pressure better throughout the day if they completed three 10-minute walks rather than one 30-minute stroll.
From the NY Times. Fitness is more generally correlated with frequency of exercise rather than duration. Intensity plays it's part but, for health, it looks like just getting out the door more often will deliver a bigger jump. Go read the whole thing.. .