WASHINGTON - Baby boomers say their biggest health fear is cancer. Given their waistlines, heart disease and diabetes should be atop that list, too.

Boomers are more obese than other generations, a new poll finds, setting them up for unhealthy senior years.

And for all the talk of "60 is the new 50" and active aging, even those who aren't obese need to do more to stay fit, according to the Associated Press-LifeGoesStrong.com poll.

Most baby boomers say they get some aerobic exercise, the kind that revs up your heart rate, at least once a week. But most adults are supposed to get 2 1/2 hours a week of moderate-intensity aerobic activity - things like a brisk walk, a dance class, pushing a lawn mower. Only about a quarter of boomers polled report working up a sweat four or five times a week, what the average person needs to reach that goal.

Worse, 37 percent never do any of the strength training so crucial to fighting the muscle loss that comes with aging.

Walking is their most frequent form of exercise. The good news: Walk enough and the benefits add up.

"I have more energy, and my knees don't hurt anymore," says Maggie Sanders, 61, of Abbeville, S.C. She has lost 15 pounds by walking four miles, three times a week, over the past few months, and eating better.

More boomers need to heed that feel-good benefit. Based on calculation of body mass index from self-reported height and weight, roughly a third of the baby boomers polled are obese, compared with about a quarter of both older and younger responders. Only half of the obese boomers say they are regularly exercising.

An additional 36 percent of boomers are overweight, though not obese.

The nation has been bracing for a surge in Medicare costs as the 77 million baby boomers, the post-war generation born from 1946 to 1964, begin turning 65. Obesity - with its extra risk of heart disease, diabetes, high blood pressure and arthritis - will further fuel those bills.

"They're going to be expensive if they don't get their act together," says Jeff Levi of the nonprofit Trust for America's Health. He points to a study that found Medicare pays 34 percent more on an obese senior than one who's a healthy weight.

About 60 percent of boomers polled say they're dieting to lose weight, and slightly more are eating more fruits and vegetables or cutting cholesterol and salt.

But it takes physical activity, not just dieting, to shed pounds. That's especially important as people start to age and dieting alone could cost them precious muscle in addition to fat, says Jack Rejeski of Wake Forest University, a specialist in exercise and aging.

Whether you're overweight or just the right size, physical activity can help stave off the mobility problems that too often sneak up on the sedentary as they age. Muscles gradually become flabbier until people can find themselves on the verge of disability and loss of independence, like a canoe that floats peacefully until it gets too near a waterfall to pull back, Rejeski says.

He led a study that found a modest weight loss plus walking 2 1/2 hours a week helped people 60 and older significantly improve their mobility. Even those who didn't walk that much got some benefit. Try walking 10 minutes at a time two or three times a day, he suggests, and don't wait to start.

"I don't think there's any question the earlier you get started, the better," says Rejeski, who at 63 has given up running in favor of walking, and gets in 30 miles a week. "If you allow your mobility to decline, you pay for it in terms of the quality of your own life."

When it comes to diseases, nearly half of boomers polled worry most about cancer. The second-leading killer, cancer does become more common with aging.

"It's the unknown nature, that it can come up without warning," says Harry Forsha, 64, of Clearwater, Fla., and Mill Spring, N.C.

Heart disease is the nation's No. 1 killer, but it's third in line on the boomers' worry list. Memory loss is a bigger concern.

"On a scale of one to 10, seven or eight," is how Barry Harding, 61, of Glen Burnie, Md., puts it. "It's more talked about now, Alzheimer's and dementia."

In fact, more than half of boomers polled say they regularly do mental exercises such as crossword puzzles.

After Harding retires, he plans to take classes to keep mentally active. For now, he's doing the physical exercise that's important for brain health, too. He also takes fish oil, a type of fatty acid that some studies suggest might help prevent mental decline.

Sanders, the South Carolina woman, says it was hard to make fitness a priority in her younger years.

"When you're younger, you just don't see how important it is," says Sanders, whose weight began creeping up when breast cancer in her 40s sapped her energy. Now, "I just know that my lifestyle had to change."

The AP-LifeGoesStrong.com poll was conducted from June 3-12 by Knowledge Networks of Menlo Park, Calif., and involved online interviews with 1,416 adults, including 1,078 baby boomers born between 1946 and 1964. The margin of sampling error for results from the full sample is plus or minus 4.4 percentage points; for the boomers, it is plus or minus 3.3 percentage points.

Knowledge Networks used traditional telephone and mail sampling methods to randomly recruit respondents. People selected who had no Internet access were given it free.