There is no question that your puppy loves to play, you can see it in his eyes as they brighten up when he sees toys that entice him to stalk, pounce, romp.
 

The most important consideration with puppy exercise is your puppy's developing skeletal system, in particular his “growth plates.”  Growth plates are soft areas at the ends of the long bones that contain rapidly dividing cells that allow bones to become longer until the end of puberty. After sexual maturity, the growth plates calcify, the rapid cell division ends and the growth plate becomes a stable and part of the bone. In puppies, this closure is completed by approximately 18 months of age. Until your puppy’s growth plates close, they are soft and vulnerable to injury. Injuries to the growth plate are extremely painful and may not heal properly or not heal in time for the puppy to grow up straight and strong. Injury to a growth plate can result in a misshapen or shortened limb which, can create an incorrect angle to a joint which can make the puppy more prone to more injuries in his future. So long walks, exercise sessions, endurance training or agility is better left until your puppy has completed his bone growth.
A good rule of thumb with exercise is five minutes of exercise per month of age twice a day. This means a four-month old puppy should get about 20 minutes of exercise at a time. Short walks, brief training sessions and play with other pups is often sufficient for young pups.  

Spay/neuter and joint diseases have now proven to be related.  This seemingly simple surgery has the capability of permanently changing a healthy puppy joint into an unhealthy one.

Excercise Tips from CairnShire