Well IMO 15 miles of running next to a cycle daily is way too much exercise for your dog.
First of all exercise should be varied so that you are not exercising the same muscle groups all the time (think of all the different exercises in a gym).
Secondly the more exercise you give a dog or a person the more it will need.
Thirdly, dogs do not get their energy from carbohydrates, they get it from fats therefore if you increase the energy needs of the dog then you must give the dog more energy via fats and/or calorifically dense foods. Too much bulk via carbohydrates will a) not provide the energy required and b) could predispose the dog to bloat.
A dog should never be RUNNING over that distance in any case; dogs will TROT in endurance situations not run or gallop!
Look at GSD they are bred to trot all day as a living fence etc.
The Ausdauerprufung test is held over 12 miles and that is the continental endurance test.
My dog competes in Working Trials and Schutzhund and I only bike him 2/3 times a week. I intersperse this with swimming, tracking, walking and other exercises and decide whether I am working on stamina, speed, strength or skills.
I believe that you are predisposing this dog to injury acute and/or chronic.