Nick Jones of Alpha Dog Behaviour discusses dog exercise
Not all dogs require a daily flogging until they come back wet and exhausted. I think it's just as easy to do too much as it is to do too little.
About 18 months is in my mind the best age to start opening the throttle more and exposing the dog to longer and more strenuous exercise. By this time the muscular and skeletal systems are well established, and able to take the strain.