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Dr. Walter Bortz doesn't countenance excuses. "Get a pedometer," he told me during a chat at last fall's AARP conference in Orlando, Florida. The former co-chairman of the American Medical Association's Task Force on Aging, former president of The American Geriatric Society and Clinical Associate Professor of Medicine at Stanford University wasn't about to waste his time hearing excuses about my having little to time to exercise. And this preeminent expert on healthy aging, who ran his 40th marathon at age 80, was equally adamant on Tuesday as he addressed a Commonwealth Club audience in San Francisco on the topic: "Is American Healthcare Threatening the Stability of the Nation?" (The answer appears to be "Yes.")

Walter M. Bortz II, M.D. has a few years to go to make the century mark but that hasn't prevented him from writing the guidebook as he goes along.

His latest (though not his latest book) is "The Roadmap to 100: The Breakthrough Science of Living a Long and Healthy Life," written with Randall Stickrod (Palgrave MacMillan, 2010).

This is a how-to book with a lot of science backing up the advice. Call it a fitness program for the price of the book (or, undoubtedly, free from your local library). Bortz, who in his Commonwealth Club speech railed against the fitness gurus who peddle pills instead of pedaling toward real fitness, offers simple advice. Get moving.

"Aging is not a disease," he tells readers. Employing a favorite metaphor comparing the life of an automobile with that of a person, he writes that the lives of both cars and people depend on four elements: design (genes), accidents (accidents, infectious diseases, crime), maintenance (diet, exercise, psychological health), and aging.

Good genes are nice, of course, but Bortz has good news for people like my friend Jack -- who is still running at 60-plus all the while worrying that he will follow his father and grandfather into an early grave -- when he notes that studies of long-lived populations show "that lifestyles, rather than any genetic factors, were the key underlying cause of longevity."

Use it or lose it is Bortz's mantra. He details how inactivity lowers glucose utilization, causes muscles to lose mass and bones to become weak, lessens the immune cells' ability to effectively attack viruses and bacteria, narrows the arteries, decreases metabolic rates, compromises the central nervous system, and leads to frailty. Although he explains all of these factors in scientific terms, he does so in clear language that we non-scientists can understand and appreciate.

"Leg strength appears to be a key indicator of frailty," he writes, in a chilling admonition to those of us with bad knees and arthritic hip joints. "... leg power, not age or disease state, is the single best predictor of a subsequent need for nursing home placement. The legs may well be the most important physical element in the aging person's body, more so than heart, lungs, and brain, because healthy legs can improve the health of all the other organs. .... The vast majority of aerobic exercise comes from movement driven by leg power." That's enough to get me off the couch and, if not running, at least walking briskly around the block.

That walk will increase my oxygen level, another major factor in health and one of "The Components of Successful Aging" that Bortz catalogs, along with lean muscle mass, good nutrition, sex, having a healthy brain, being necessary, and movement, the last which "is both cause and effect, an enabler of health and the result of health."

He offers advice on how to achieve this lasting health and, with it perhaps, a longer life, in simple, though not simplistic terms, backing each up with evidence from various scientific studies. This isn't an exercise manual or recipe book. Bortz is not telling us exactly what to do so much as he is explaining why this advice is important: "If we have learned anything from the science of the past decade or so, it has been the overarching theme that health -- and longevity -- is mostly a matter of choice, not fate."

Among the "14 Healthy Choices" he offers as a summation of the book's themes, is to "Move! ... Movement is life." And, he told his Commonwealth Club audience: "It's never too late to start but it's always too soon to stop."

Now 81, Bortz has finished his 41st marathon and will hit the streets of San Francisco July 31 with his wife, Ruth Anne, to run the San Francisco Marathon. Having finished his seventh book, "Next Medicine: The Science and Civics of Health" (Oxford University Press, 2011) and begun his eighth, titled "What's Good About Being Old", Bortz is nowhere near stopping. If, as it appears, he's taking his own advice, then "The Roadmap to 100" is as good a how-to book as any you'll find to help you get the rest of your lifespan up and running.