I run with my dog five days a week (I let him have the weekends off). I started running with him when he was a year old, and I followed a modified Couch to 5K running program designed for humans in order to condition him. After that we increased our mileage by about 15% a week. His longest run to date is 13 miles, but generally we do just 6 - 10 miles at a time, so his highest mileage week is 50 miles.
Before we began getting serious about mileage, I made sure that his hips and elbows were okay via x-ray.
If you run in town or city, make sure you keep an eye on his paws. I run trail, so it's dirt and not really something I need to worry about. But in the summer you need to worry about hot pavement, and in the winter you need to watch for injury to the paws from salt or sharp ice.
The only times when I do not run with Niko, aside from his rest days, are when it is frigidly cold and single digits, or when it is excessively hot and over 75 degrees. In the summer I run with him early in the morning to avoid the heat.
Running keeps him nice and trim, and he is a joy to run with. He is the reason why I am so consistent with my running, because I know how much he enjoys it. I never skip a run any more.
Niko: American Showline GSD 5 years old
Rosa: American Muppet Dog (GSD/Border Collie mix) 5 years old