The old adage* is true, according to researchers from Michigan State University.
Thanks to Tara Parker-Pope's post in today's NY Times, "Forget the Treadmill. Get a Dog:"
[A]mong dog owners who took their pets for regular walks, 60 percent met federal criteria for regular moderate or vigorous exercise. Nearly half of dog walkers exercised an average of 30 minutes a day at least five days a week. By comparison, only about a third of those without dogs got that much regular exercise.
The researchers tracked the exercise habits of 5,900 people in Michigan, including 2,170 who owned dogs. They found that about two-thirds of dog owners took their pets for regular walks, defined as lasting at least 10 minutes.
And it turns out, if you walk your dog, you're likely to be otherwise physically active in sports and gardening.
On average, [dog walkers] exercised about 30 minutes a week more than people who didn’t have dogs.
*Cursory search suggests that Dr. William Osler, one of founders of Johns Hopkins, was first to say this.