I start puppies at 6-8 months old running short distances. I never run less than 3 miles, so that's where I start them out. I've always hear people say a year to start, but with medium sized dogs like pit bulls, I've never worried about waiting that long. With a giant dog like a Dane, I'd wait till they were about two!
As for how long they can run-much longer with you can
Just be aware, as Jmann said, of the fact that they don't cool as efficiently as humans. In summer, in warm climes, stick to early mornings and evening time. I notice my dogs have a decline in endurance during the hottest months, but they can still run whatever distance I'm going.
I've also noticed that the super thick, heavily muscled pit bulls are much less athletic than the leaner built ones. Lugging all that muscle mass around is tiring, I guess :roll: My 38 lber has amazing endurance and speed.
Have fun with your pup! Dogs make the best running partners-no whining about turning back or being tired or having sideaches! Plus, they never say they can't or don't want to go for a run. Very motivating