I have read in my yorkie books that you can condition a yorkie to walk up to 9 miles a day. Of course you need to increase the walk time gradually, and not all yorkies will comply (Tina just sits down after about 3 blocks
), but if you start working up to it you will know what your pup can handle. Tbone, about the same size as you described, can go for over an hour at a time, and still be worked up enough to play afterward!