Aging expert Walter Bortz, widely known as the "running doctor" who completed 45 marathons while advocating the correlation between exercise and longevity, can no longer run.

At 88, the retired Stanford University professor, physician and author of titles such as "Dare To Be 100," "Living Longer for Dummies" and "Roadmap to 100" is personally experiencing some of the health trials he's spent a career studying and writing about.

During a recent interview at his Portola Valley home, seated in a comfortable chair with a walker by his side, Bortz shared what it's like to be a senior.

Advanced age "used to be something I observed in other people, and now it's happening to me," Bortz said. "So instead of being objective, it's subjective."

Bortz, who until recently was an active runner, is scheduled to undergo back surgery to address spinal stenosis.

He holds out hope that the surgery might enable him to get back to the running routine he loves so much — or at least to walk comfortably. He considers the surgery as a minor disruption in his quest to stay true to one of his cardinal tips for successful aging: "Don't slow down."

He remains firmly convinced that regular, sustained vigorous exercise is key to living 100 years in good health.

"Use it or lose it," he said. "My mantra is '100 healthy years.' Our birthright, our warranty is 100 healthy years if we don't screw it up."

With his engagement and curiosity undiminished, Bortz said he continues to travel and lecture and is at work on his next book — about aging — with the tentative title of "Aging Is Negotiable."

Running — around Stanford's Angell Field, up Portola Valley's Windy Hill, and on tracks and paths throughout the world — has been one of the great passions of his life. But he admits he might have overdone it on the marathons.

"The Greeks said, 'Everything in moderation,' and I was not moderate. I think I just wore (my legs) out, just gone from too much use," said Bortz, who ran the Boston Marathon in 2010 to celebrate his 80th birthday and then again in 2013.

"I never had any distinction as a runner — I was once interviewed by PBS for coming in last in the Boston Marathon — but I love to run," he said. "I'm terribly upset when I see runners running and I can't do it. It bothers me."

On death and dying, Bortz aligns himself with the message of surgeon and writer Atul Gawande in the book "Being Mortal": People should consider their deepest values and strive to maintain them as much as possible even in the final weeks and days of life.

"We want to die actively, not inactively," Bortz said. "My wife died here in this house after falling out of bed and hitting her head. She had advanced Alzheimer's. No pain, no tubes, no loneliness."

He was delighted to share that in the past year and a half he has found new love and companionship with Jeanne Kennedy of Palo Alto, whose photo sits among the many family images in his memento-filled home. (Kennedy, contacted separately, confirmed that she is equally delighted.)

Bortz began his career in the 1950s, practicing medicine in Philadelphia with his father, geriatrics trailblazer Edward Bortz, who chaired an early White House Conference on Aging and helped found the AARP.

"I was an only child, and I worshipped him," he said of the relationship.

At his father's death in 1970, Bortz, then 40, took up running and exercise to deal with his overwhelming grief.

"I knew exercise was the best treatment for depression, and I was devastated, clinically depressed," he said.

That same year, Bortz and wife, Ruth Anne, left Philadelphia and moved their four children to California, buying the home in Portola Valley he still occupies. Bortz began practicing at the Palo Alto Medical Clinic, a precursor to today's Palo Alto Medical Foundation.

"When I joined the clinic they said, 'You are our anointed gerontologist,' and I loved it," he recalled. He served as a physician for local senior communities Channing House, the Sequoias and Casa Olga. He made house calls and began teaching Stanford medical students, which he has continued to do.

Among his proudest achievements, he said, was chairing a board that got a senior center built in East Palo Alto. He also took on national leadership roles in the American Medical Association, the American Geriatrics Society and the Diabetes Research and Wellness Foundation.

Though he'd published extensively in medical journals, Bortz never considered writing for a general readership until he befriended Norman Cousins, then a nationally prominent writer and editor in the 1980s.

"Cousins was brilliant — he espoused the idea of attitude and wellness," Bortz said. "He told me, 'Walter, you've got to stop writing these scientific articles and write a lay book.' He wrote a wonderful blurb for my first book and got Bantam to print 75,000 copies."

That first book, "We Live Too Short and Die Too Long," came out in 1991. Over the next two decades — on top of a busy medical and teaching schedule — Bortz produced seven more.

"The writing was interspersed," he said. "When I was writing a book, I'd write a chapter a month. I was disciplined."

Recurring themes were aging, diabetes and reform of the health care system.

In "Next Medicine," published in 2011, Bortz argued that financial interests have "eroded the values of the medical profession and placed profit before human well-being."

Heart disease, for example, "is widely treated with drug interventions and invasive surgery — both of which are extravagantly profitable for pharmaceutical giants and hospitals. But daily exercise and a healthy diet can help prevent heart disease and can be obtained by patients essentially for free."

Until the "medical-industrial complex" drops its "vested interest in keeping Americans sick ... medicine will fail to effectively address the leading cause of disability and mortality today: chronic diseases like diabetes that are largely preventable," he said.

Bortz advocates reforming health care by boosting incentives for healthy lifestyle choices throughout the system.

"I went to talk to health insurers in Minneapolis about five years ago, and I said, 'Why don't you preach health? Give everybody who registers for AARP a step-counter. For every 25 steps you take, you can save a penny on health care costs,'" he said.

As he approaches his 89th birthday, Bortz said he's sticking to his eight tips to age "successfully" laid out in his first book, including: "Set goals and accept challenges that force you to be as alive and creative as possible."