So, what’s it worth to lace up those sneakers and break a sweat for about 30 minutes a day? About 3.5 extra years of life, on average — and about 4.2 additional years for those willing to step up the intensity or put in closer to an hour a day of brisk walking or its equivalent, according to a new study.

Even for the severely obese — those with a body mass index above 35 — exercising for about 2.5 hours a week at moderate intensity or for 75 minutes at vigorous levels puts average life expectancy a notch above that of a normal-weight person who is sedentary, the research shows.

It’s no surprise that exercise is good for you and will help you live longer. But the study published Tuesday by the journal PLoS Medicine sounds a loud wake-up call to “healthy weight” couch potatoes who believe their good BMIs will ensure them a long life.

Even for people with a BMI between 20 and 25, those who told researchers they were physically inactive were far more likely to die in the next decade or so than were overweight or obese exercisers. Among the study participants over the age of 40, the sedentary were almost twice as likely to die during the course of the study than were participants who were highly active.

“This finding may convince currently inactive persons that a modest level of physical activity is ‘worth it’ for health benefits, even if it may not result in weight control,” the study authors wrote.

The results also offer clear evidence that exercise can offset some of the longevity loss that comes with past or continued tobacco use or a history of cancer or heart disease. Among those groups, getting even a modest amount of physical activity restored between 2.5 years (for current smokers) and 5.3 years (for cancer patients) of lost life expectancy. And getting more — or more vigorous — exercise added even more time.

The levels of physical activity that yielded such benefits were modest. The authors of the study observed an uptick in life span even among those whose physical activity fell short of what’s recommended by the U.S. Department of Health and Human Services and the World Health Organization.

Dr. Francisco Lopez-Jimenez, a Mayo Clinic cardiologist who was not involved in the research, called the study “very conclusive” and said its enormous scale — it culls data from six major study populations totaling more than 632,000 people — bolstered the strength of its findings.

“We have to set priorities with patients,” Lopez-Jimenez said. “First and foremost is to get sedentary obese people to become as active as they can and not to use their weight as a measure of their success. Sometimes, we tend to focus too much on the weight issue and too little on the exercise part of it.”