About 15 minutes later, he was walking back to his friends when the first bomb went off.

NPR: Fla. Runner Gets Photo Of Suspect Fleeing Marathon

The 2008 Physical Activity Guidelines for Americans released by the Department of Health and Human Services recommends that adults age 18 to 64 engage in at least 2.5 hours of moderate-intensity aerobic exercise (brisk walking, biking) or an hour and 15 minutes of vigorous activity (like running) each week.

CNN: Is a shorter but harder workout better?

The Lancet study, based on a review of more than 400, 000 people in Taiwan, showed 15 minutes per day or 90 minutes per week of moderate exercise, such as brisk walking, can add three years to your life.

BBC: 15-minute daily exercise is 'bare minimum for health'

The study exposed 40 healthy participants with a mean age of 27 to two minutes of slow split-belt walking, then to a 15-minute period in which one belt went much faster than the other so that participants began to drag one foot, simulating a limp.

WSJ: Split-Belt Treadmill Therapy for Stroke Patients

Jim Stone, executive director of walking-advocacy group WalkSanDiego, says he has found Google can say a mile will take anywhere from 15 to 30 minutes, depending on the details of the route.

WSJ: Timing an Urban Stroll Isn't a Walk in the Park