I've been wanting to do this for a while, both to give me some much-needed exercise, and to give my Australian Shepherd some exercise as well.
From what I've found, there's a couple thoughts on the subject:
-buy a special attachment that bolts to the rear triangle and holds the dog's leader at a level near the axle, where it has little mechanical advantage and is subject to the master's full weight to resist it. Probably not an issue if you're a thin female with a 180-pound Mastiff along for the run, but I don't have that problem. My buddy is an Australian Shepherd, and I outweigh her by a fair margin. Ok, a more than fair margin. Shaddup. The examples I've seen run about $60, like the BikeDog (which it seems I now can't find the link to)
-similar to the above, except attaches to the seat tube. The WalkyDog is an example of this, about $50. More mechanical advantage for the pup, but it's not likely to be an issue with my riding partner and myself. The biggest thing I have against this is the $50- I have a welder, and regularly fab stuff with webbing (slings and similar), shouldn't be a problem to make the soft part). More to the point, it's been out of stock for at least the last year, near as I can tell. It looks like it's just a dressed-up pneumatic QD fitting and a post. I may have to take a look around Lowes and see what I can cobble up.
-use a standard 6'foot leash. Put the loop around your right bicep, curl it around the headset, and then to the dog. I see lots of opportunity for problems here: the dog has mechanical advantage against the pivot point (tire/ground interface) of the bike/rider assembly. Also there's an issue with the lead possibly fowling in the chainring or at least being a hindrance to the rider's leg. I know a couople folks that've had problems with otherwise-calm dogs bolting at the sight of a squirrel and causing injuries with this sort of setup. I know Xavier doesn't have a problem with this arrangement- I'm just not comfortable with the idea. While my Aussie is very calm (I can walk her without a leash and she has never left my side), I don't want to go this route.
Anyone have other thoughts or input?