A couple are walking home with their dog at a steady 4mph. With 6 miles to go the dog is let off the leash and runs all the way home at 6 mph. The dog immediately turns and runs back to the couple at the same speed; and upon meeting them it turns and runs home again. It continues to run back and forth at the same speed until the couple reach home. What distance has the dog run since being let off the leash?