Janet Rapp strode briskly down a paved path through the city zoo, waving at friends and stopping briefly to greet emus she knows by name.

The 71-year-old retiree starts each morning this way with a walking club.

"I'm obsessed," she said. Not only does it ease her joint pain, "it just gives me energy …And then it calms me, too."

Medical experts agree that walking is an easy way to improve physical and mental health, bolster fitness and prevent disease. While it's not the only sort of exercise people should take, it's a great first step toward a healthy life.

"You don't need equipment and you don't need a gym membership," said Dr Sarah Eby, a sports medicine physician with Mass General Brigham, a not-for-profit healthcare system in the United States. "And the benefits are so vast."

What does walking achieve?