You can go as far as your dogs condition allows you to go. Start out with short distances and work up to longer distances. Bring mucho water, a towel, and check your dogs feet, a lot.
When the dogs tongue is really large, sticking out, and the end is rolling up, time to stop because the dog is tired.
Common sense plays a large role, don't over do it, and don't expect to much out of your dog. Slow and easy. Eventually I have no doubt the dog could trot 12 miles.