It's wonderful that so many people are trying to get fit by walking, running and doing many other activities. But trying to train or run while "walking" your dog can have a downside for him. I see so many people, usually guys but not always, who keep running and running, often with an exhausted, panting dog behind them. Sometimes the owner's pace is too much for his canine companion. Once the two legged fitness guy or gal gets going they may forget or neglect to check on Fido struggling to keep up. Add to this that the four legged friend behind them is often being pulled by the neck and needing to stop for his bathroom needs, the supposed purpose of the outing to begin with.
While many dogs, especially young ones, love to run, please make sure to check that the dog's "toiletries" are taken care of first and that he/she is capable of keeping up with you before you set out to practice for your next marathon. Otherwise, happy running!