Not relevant to the OP, but I will note that I was recently trying to work out a way to exercise a dog who needed to be kept on leash but also get a lot of exercise and tried two devices -- the Walkie Dog and the Springer Dog Exerciser (both searchable through Amazon among other places). I found the Walkie Dog marginally functional and the Springer pretty useful (I replaced the "attach the dog using this rope" that they include with the product, with a short dog couple that had snaps on each end, and attached it to a nylon harness she wore). To be clear, I (a) worked with the dog so she grasped the basic concepts involved and talked with her when we were out using a few simple verbal cues to let her know if I wanted to slow down or speed up and (b) picked routes carefully -- this wasn't an "I'm going out and I'll take the dog along" thing, it was an "I'm going out with the express purpose of exercising the dog safely using the bike."
It worked pretty well, and she liked the running she could do thanks to this (she has since been adopted by another person, which is why I'm using the past tense -- not the right dog for me long term, but a lovely dog while I had her and provided I could get her enough exercise :) !).