I have a 90 lb. 1 1/2 year old mutt. He's a bulldogish/mastiffy kind of mix . . . not overly hyper (like my friends lab mix), but not a lazy dog either.
The other day I had him follow me on a ~ 10-15 mile mountain bike trip. I brought along plenty of food and water for the both of us, and we stopped and rested regularly. It was a relatively cool day.
On the uphill sections of our trip, he was usually ahead of me, as I was crawling slowly on my bike. But on the downhill sections (when I was crusing along at a pretty fast pace), he was starting to look a little beat. And towards the end of the trip, I was starting to get a little worried that perhaps I was running him too much (although he wasn't showing any serious signs of fatigue, just heavy panting).
The next day, however, he was moving around like he was a 100 year old dog. [

]
So how much running can a medium-large dog actually handle?