Doug von Koss was born in the Depression and raised on the banks of the Mississippi River in a houseboat his father built from salvaged lumber. In the 1960s he settled in San Francisco, where he and his wife, Clydene, raised their son and daughter. He made his living as a stagehand, theater carpenter, light board operator, and set dresser for films like George Lucas''s Return of the Jedi. He''s now eighty-five, tall, elegant, and commanding. Widowed for a decade, he lived in a neat, rented bungalow on a hilly San Francisco street. In his fifties, while he was working as prop master for the San Francisco Opera, he led a workshop in mask making at a men''s conference in the redwoods of northern California. The men, who''d just met, nervously labored over their masks in silence, with pinched faces and little joy. The poet Robert Bly, one of the conference organizers, nudged Doug''s arm and said, "Get them singing.

" Doug drew the men outside. After twenty minutes of belting out camp songs under the redwoods, the men loosened up, started talking with each other, and returned to sculpting their masks with abandon. Ever since, Doug has been flying around the country, helping groups build community by leading them in traditional songs, chants, and poems that he''s gathered from cultures around the world. Not long after his seventy-ninth birthday, Doug found the steps up to his front door growing steeper by the day. At first, he brushed off his fatigue and breathlessness as normal aging. Then one midsummer afternoon, as he was pushing a shopping cart through the supermarket, he felt light-headed, dizzy, and short of breath. He trundled over to the one place where he could sit down: the doit-yourself blood pressure machine near the pharmacy. He doesn''t remember now whether his reading was too high or too low, only that it wasn''t good.

The next morning in a medical building downtown, his doctor stopped in the middle of recording Doug''s electrocardiogram and called an ambulance. EMTs took Doug down the elevator on a gurney. Twenty four hours later, in a cardiac lab at a nearby hospital, doctors inserted a small tubular metal cage called a stent into an artery leading to the heart''s largest blood vessel. "One of the main vessels was plugged," Doug said. "I could have gone belly up." He''d been millimeters from a heart attack. The stent pushed aside a clump of fatty plaque, propped open the artery walls, and increased the flow of oxygen-rich blood to Doug''s heart, body, and brain. He found it almost instantly easier to climb his front stairs.

"Life became incredibly sweet," he remembered. "I could stop and look at a tree, look at a flower, and really see it. I felt really alive, and at the same time very fragile." The stent, he sensed, was a temporary reprieve. Why, he wondered, had fat, cholesterol, and calcium congealed in his arteries? He didn''t smoke or drink, never touched bacon, and rode his bike in Golden Gate Park three times a week. "But I got the message," he said. "Pay more attention, Doug. There''s a line between disease and optimum wellness, and you''re sliding into disease.

" His hospital offered a four-month program of intensive cardiac rehabilitation, paid for by Medicare. Three times a week at a rehab center, he strapped on a heart monitor and pedaled a stationary bicycle while a physical therapist helped him gradually increase his heart rate. A dietician nudged him toward the Mediterranean Diet--less meat, dairy, sugar, and packaged foods; more vegetables, whole grains, olive oil, fish, and fruit. That, combined with more strenuous exercise, halved Doug''s risk of having a heart attack or dying within five years--and more importantly, it substantially extended the years he will probably spend thriving. When the cardiac program ended, Doug joined a Y and started running on a treadmill three times a week. At eighty-two, he began lifting weights. "I looked around the gym and saw men and women, whom I knew were as old as I was, walking very vigorously," he said. "I wanted that, too.

" He built muscle and improved his balance-- crucial capacities, given that muscles naturally wither with age, agility lessens, bones grow brittle, and independence can be devastated by a fall. "It started a great wellness loop," Doug said. "More exercise, healthier eating, better sleep, and an improved sense of wellbeing." At a recent checkup, his doctor said, "Don''t change a thing." The health stage I call Resilience, sometimes called "young" or healthy old age, is a time when you still have the physical capacity to reverse substantial health problems. Most people in the Resilience stage are in their fifties, sixties, and early seventies, but some are exceptionally athletic older people, like Doug von Koss. Length of life is impossible to predict precisely, but people at this stage usually have at least another decade left to live. This is the time to take inventory, build reserves, and assess what needs shoring up.

The major threats to your future well-being will be: physical weakness, isolation, heart disease, lung disease, diabetes, and dementia. You can build bulwarks against them--and prolong your time in Resilience--by exercising, eating better, and widening your circle of friends and passionate interests. Lifestyle habits-- especially smoking, being sedentary, eating poorly, and drinking too much alcohol--are responsible for 70 percent of the degenerative diseases that make later life difficult. Change these habits, even after the age of fifty-five, and you can cut your health risks as much as sevenfold--a better payoff than almost all drugs. I don''t mean to suggest that food asceticism and strenuous exercise will ward off death and decline forever. They won''t really make you younger next year, though they may keep you happier, stronger, and more functional. Given that our bodies age at the cellular level in more than five thousand specific ways, there''s little point in strengthening physical muscles without developing the spiritual and social strength to cope with the inevitable loss of powers, and with death itself. But before you must accept the things you cannot change, you can seize the time to prepare for what''s ahead, and to change the things you can.

BUILDING RESERVES In developed countries, few people die of disease in the first half of life. Most early deaths result from accidents, violence, drug overdoses, and suicide. In late midlife, the picture changes. Cancer becomes a major cause of death in the mid-forties and continues to climb throughout the fifties and sixties. Deaths from heart disease rise in the sixties and seventies, from lung disease in the eighties, and from dementia in the nineties. All cause physical suffering long before they kill, and all are profoundly shaped by how you live. I suggest you begin by doing what requires the most of you and the least of medicine. The most effective first step (other than quitting smoking) is to walk energetically every day.

People over sixty-five who do so increase their lung capacities, get more oxygen to the brain, and expand the size of the hippocampus, a brain organ crucial to memory. As a side benefit, walking around malls, Farmers Markets, and to downtown coffee shops amplifies social connections, another delightful way of improving health, brain function, and happiness. Most of this is not news. But if you''ve forgotten the deep pleasure and self-confidence that can follow half an hour or more of aerobic exercise, especially in nature or with a friend, consider reacquainting yourself. Even late in the game, getting more active has huge health benefits. Exercise becomes more challenging as joints grow creaky and minor injuries heal more slowly. Improvise, adapt, and overcome: get moving in any way that makes you break a sweat and gives you joy. Many people find delight in ballroom dancing, biking, or swimming; others find it easier to get started--and to keep going--by scheduling a regular exercise date with a friend.

If your feet or knees hurt, consider upgrading shoes or improving your posture or gait with the help of a podiatrist, a physical therapist, or a practitioner of an alternative approach, like Feldenkrais or the Alexander Technique. Stay flexible and be willing to substitute a new activity whenever one falls by the wayside: if you can''t run anymore, try water aerobics; if you lose your partner, explore group activities like Greek or country line dancing. No matter what happens, keep going. The body''s capacity to heal, even at this relatively late date, is astounding. Tom Murphy, a former Associated Press journalist who''d once run a marathon, was sixty-two when he was diagnosed with diabetes. He''d been working a stressful and unsatisfying job and, he said, had "fallen into my mom''s habit of eating mostly cookies and ice cream, frozen pizzas, Danishes, and lots of bread." He took a new job and moved from the San Francisco suburbs to rural Mendocino county. By the time he met his new primary care doctor, he weighed 225 pounds and had a trifecta of late-life warning flags: high cholesterol, high blood pressure, and high blood sugar.

His alarmed physician recommended he see a cardiologist immediately and start taking a cholesterol-lowering statin, a blood pressure-reducing diuretic, and the blood sugar-lowering drug metformin. Tom looked at his friends and family, many of whom were already on these drugs, and saw his own future. "I have a friend who went blind from diabetes, another who can''t walk, and a third who died of a heart attack," he said. "All could have changed their diets in their fifties, but waited too long. I wasn''t going to make the same mistake." He took blood pressure medication to lower his stroke risk, but asked for a grace period before adding o.