Scout
I have a working lab, and he's done upto 19 miles with me quite happily.
We started doing 1-2 mile laps of the park when he was just over 12 months, and built slowly from there. He absolutely loves it when we're out... and he is a much more chilled relaxed dog when he gets plenty of exercise, so it's a win-win
So, my advice would be to start slowly, and enjoy it. The sensation of running with your dog is fantastic......