(10-14) 11:25 PDT SAN FRANCISCO -- Earlier this week a woman made national headlines - and made an entire country feel like lazy good-for-nothings - when she finished the Chicago Marathon while 39 weeks pregnant, and gave birth hours later.

But take heart, couch potatoes. You don't have to run a marathon, or run at all, to reap impressive health benefits from exercise.

In fact, if the average adult walked 6 1/2 hours a month - or about the same amount of time it took that pregnant athlete to finish the marathon - he could add three years to his life, according to a new study.

"It doesn't take a lot. You don't have to be super intense, like a triathlon or an Alcatraz swim," said Dr. Moshe Lewis, a California Pacific Medical Center physician who specializes in pain management and sports medicine. "Only 15 minutes a day, that's not much. It's great for people to know that walking around, doing some stairs, that's getting a cardiac regimen going."

Doctors and public health experts have been losing ground against the tide of unhealthy Americans who refuse to exercise. Only about a third of U.S. adults exercise regularly, defined as three to five times a week of moderate or vigorous physical activity, according to the National Health Interview Survey conducted last year. Another third of adults get no exercise at all.

Forget marathons, or even the occasional charity 5K - doctors are having a rough time just getting people to walk around their neighborhood a few times a week.

Studies say it's so

That's a shame, they say, because studies show that those walks could significantly lower their cholesterol, blood sugar levels and blood pressure. They could even help people stop smoking and eat better.

One study of more than 400,000 adults in Taiwan - the same one that showed minimal exercise can add years to someone's life - found that 15 minutes of moderate exercise, six days a week, reduced the risk of death by 14 percent compared with people who aren't active at all.

More for you News Short bouts of exercise pay huge dividends

U.S. guidelines suggest at least 30 minutes of exercise five days a week, and doctors generally agree that more exercise is better than less. Still, just getting patients out the door for a few short walks a week could do wonders.

"The low-hanging fruit, in terms of really getting benefit from exercise, it's the people who aren't doing anything at all. If we could just get them to walk, the health effects would be astronomical," said Dr. Robert Sallis, a Southern California family doctor who helped develop a campaign for Kaiser Permanente to get members walking regularly.

On your feet

Similar walking programs are taking off around the country. Kaiser's campaign, called Every Body Walk, started in January, and this month the American Heart Association introduced a walking-focused website that lets users find paths in their neighborhood or map new ones, log their exercise and buy training gear.

Holly Potter of Berkeley gets most of her regular exercise from walking - parking her car far away from her office at Kaiser in Oakland, for example, and walking her son to school once or twice a week. Once a month, she organizes a walk-to-school day and invites other parents to join.

She would like to get more exercise, but as a single mom with a full-time job, finding the time can be a challenge, and walking is easiest to fit into a busy day, she said.

"It's heartening to know that the research shows that getting that little bit really makes a difference," Potter said. "I try to get a little bit in every day."

Walking isn't exactly a new activity when it comes to encouraging people to exercise. For years there's been growing support for simple exercise routines that suggest things like going for a 15-minute walk during the lunch break and another 15-minute walk while the kids are at soccer practice.

What's changed, doctors and public health experts say, is that large, formal studies now offer definitive proof that even that little bit of exercise can have major payoffs. In the Taiwan study, people who did the equivalent of a short, "brisk" walk five times a week had lower rates of cancer, heart disease, stroke and diabetes than people who got no exercise.

Lowering the risk

A study published by the American Heart Association in August found that people who exercise half an hour a day, five days a week, have a 14 percent lower risk of heart disease than people who don't exercise. Public health experts weren't shocked by the finding, but it was reassuring nonetheless.

"We've been telling people for years to just walk 150 minutes a week. That seemed reasonable, but we didn't really have any hard data to back it up," said Dr. Matthew DeVane, a Walnut Creek cardiologist who is president of the American Heart Association's East Bay board of directors. "Finally we looked at it, and it does seem like you get a ton of benefit by doing that minimal amount of work."

And it's a lot easier to get someone out the door for, say, 15 minutes a day, than to tell them they need to be running marathons if they want to stay healthy, say many doctors, adding that they sometimes run out of patience with the myriad excuses patients make for avoiding exercise.

Then again, there's some concern that patients are short-changing themselves if they get too focused on short bursts of exercise. After all, if 15 to 20 minutes a day is good, double that is even better. The Taiwan study found that for every additional 15 minutes of exercise a day people got, the risk of death decreased by another 4 percent.

"Some exercise is better than none," Lewis said. "But I see the 15 minutes as just a starting point to get to what people typically should be doing, which is 30 minutes of brisk aerobic activity at a minimum."