In the wake of the beautifully run Boston Marathon, let’s think about how many years the habit of running can add to our lives. Three. And chances are, they’ll be three good years, lived in at least relative health. Writing in the New York Times, Gretchen Reynolds suggests that “running may be the single most effective exercise to increase life expectancy.” Reynolds cites a three-year-old study by researchers at the Cooper Institute in Dallas that found that as little as five minutes of daily running could help us live longer. Really? It wasn’t exactly clear, so Reynolds offers a new study that used the same data, and found that running, just about any running, dropped a person’s risk of premature death by almost 40 percent. The researchers calculated that running returns more time to people’s lives than it consumes, and that just two hours per week of running could expect an increase in life expectancy of 3.2 years.