With your caveat of 20 miles/week with 4-5 miles sessions I would pay careful attention to WHERE you run. If you're running purely on pavement and the dog can't run on anything but pavement you run the risk of pounding his or her joints to pieces. Make sure that they can run on bare ground, lawns, ditches, etc. to protect their joint health. Lucy has the most fun trail running with me. It's an absolute ball!
