My White Shepherd Oz runs all marathon training with us, he can easily run the distance and more - in fact he does, running in front and back to us, up banks and down, probably does about 1/3 more mileage! He's also exremely fast over short distances, we've had him timed at 3.8 secs over 60m. All of our shepherds except for one have been able to run massive distances and Ozzy's predecessor Snowy did the Frankfurt marathon years ago, even got his own finisher's medal and pictures in the running mags.
But a word of caution - before you let your dog run any distance, it's wise to have X-rays done at least of the hips, preferably also the elbows, to rule out any possible later disfunction that may be aggravated by running.
