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 431,479 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.”

It took as little as the metabolic equivalent of a 10-minute daily walk to start extending one’s life span, the study found. Those who adhered to recommendations to get at least 2.5 hours of moderate-intensity exercise — or 1.25 hours of vigorous exercise — per week reaped gains of roughly 3.5 years. While those with a greater commitment to physical activity continued to increase their average life spans, the authors found that the resulting increase in life span slowed as participants’ exercise levels rose beyond twice the recommended minimum.

The latest study adds to mounting evidence that a sedentary lifestyle may trump obesity as a corrosive influence on health. In recent years, researchers have found that exercise, even when not accompanied by weight loss, powerfully affects a range of risk factors for cardiovascular disease and Type 2 diabetes.

Many clinicians and researchers complain that this message — that while it’s ideal to be lean, it’s possible to be fat and fit — has gotten short shrift amid the national furor over obesity. Our widespread laziness, they say, is at least as worrisome as the fact that 1 in 3 Americans is obese.

“We have to get people to understand that it’s not all about weight,” said Dr. Robert Sallis, a sports medicine specialist with Kaiser Permanente in Fontana who has spearheaded the Exercise Is Medicine initiative under the auspices of the American College of Sports Medicine. “Not everyone can lose weight. But everyone can get fit.”

While maintaining a normal weight and exercising are both important, physicians and public health officials may do more to improve the nation’s health by recommending that their sedentary patients get up and get moving, said study leader Steven Moore, an epidemiologist at the National Cancer Institute.

“You can’t lose 30 pounds tomorrow,” he said. “But you can start exercising.”

melissa.healy@latimes.com