I'm sure this is breed dependent, but what kind of distances and speeds are appropriate for dogs? I've heard say that dogs don't do well running long distances at a constant pace. I don't know how true this is or isn't. Don't sled dogs in Alaska/Canada run incredible distances for days on end? What does that do to their life expectancy?

I have an Australian Shepherd. I've walked her daily since she was 10 weeks old. We started out slow and short and eventually built up. When she was about 9 months old I started running with her at about a 12 or 13 min/mi pace, but doing 3 - 4 miles. She's now a year old and I'm looking to train for my first 1/2 marathon. Can she train with me? How far can I run her?