Agility people know who Silvia Trkman is. She's a top agility competitor in the world. I like to read her web page because it covers everything on dog training, pretty much. Now, keeping in mind that she has a smaller breed of dog (Pyrenean Shepherds), I still think her philosophy on exercising puppies can still be applied. I think it's well worth reading her entire website, even if you aren't interested in agility.
"When people ask me when good old Lo will deserve retirement, I always tell that good old Lo certainly deserved to never be retired. She is in agility since her 7th week, she is over 10 years old now and still easily beats much younger dogs on grass. "
Go to "Our Training" and scroll down to "Agility is good for dogs."