Thanks for your thoughts; and you are obviously a person who pays attention to the needs of your dog if they become tired. But unlike humans who can say "hey, stop, I am tired" a dog, has no ability to say that. Sure, they can stop and lay down, especially when walking on a leash, but I have yet to see a dog stop and lay down when tired, when someone is pulling them alongside a bike. On several occasions, especially hot days, I have seen dogs being run alongside a bike and they look like they are ready to drop.
And yes, as many dog trainers as there are around, there is just as many different opinions in regards to training a pup to walk also.