It was a few minutes past 10 on Sunday morning when Lloyd Kahn crested a hill known as Insult.
He was shirtless and mustachioed, with a receding shock of white hair, as he trotted up a brambly trail six miles into the Dipsea, the storied 7.5-mile race from Mill Valley to Stinson Beach that is said to be the country’s oldest trail race.
He had already conquered the 672 stairs rising through the forests above Mill Valley, dropped on jelly legs into Muir Woods, pushed through trails choked with poison oak and survived the steep climbs known as Dynamite and Cardiac. Now the course was about to tip downhill toward the finish line.
Mr. Kahn is 91.
A legendary apostle of homemade architecture and all things D.I.Y., Mr. Kahn has amassed an ardent following by teaching people how to make something out of nothing. He is a writer, builder and publisher whose work became a handbook for the counterculture of the ’60s and ’70s and continues to inspire new generations of back-to-the-landers. His 1973 opus, “Shelter,” was a world tour of vernacular structures: tree houses, wigwams, yurts, Earthships and other dwellings made by hand. He was also the shelter editor of the “Whole Earth Catalog,” the compendium of how-to diagrams and product listings that Steve Jobs likened to a proto-Google.
In recent years, Mr. Kahn has become a model for something else, too: growing old with vim. On Substack and Instagram, a combined 37,000 people follow his dispatches on travel, movies, music, building, exercise, aging and whatever else catches his attention.
“My posts on aging are the most popular by far,” he said a few days before the race, sitting in his hand-built, skylit workshop in Bolinas, once the headquarters of Shelter Publications. “As the baby boomers get into their mid-70s and 80s, they want my perspective. But I don’t want that to be the main thing that I’m known for.”
The workshop sits on a half-acre parcel he bought for $6,500 in 1971. He built its scattered dwellings with his late wife, Lesley Creed, using lumber recycled from old Navy barracks on Treasure Island and windows salvaged from chicken coops.
The Dipsea, with its rough-around-the-edges and faintly anti-establishment feel, is his kind of race. “The Dipsea is a romantic race, really,” Mr. Kahn said. “It’s so old, and it’s got a lot of lore that comes with it.”
The race began in 1904, more than 30 years before Marin County was connected to San Francisco by a bridge across the Golden Gate strait, when one man had the idea to race another from the Mill Valley train depot to the Dipsea Inn, located on the sand of what later became known as Stinson Beach. There was no formal course then, just wild terrain and a few trails winding around the flanks of 2,571-foot Mount Tamalpais. The next year, the two-man contest became an annual affair.
From the outset, two features defined its spirit. One was the absence of a precise route. Runners could choose their own adventure, weighing a shorter, steeper climb against a longer way around. That strategy persists, though with serious limits: Runners now wear tracking devices and are allowed only on designated shortcuts.
The other is the race’s handicap system. Unlike most races, the Dipsea does not simply send the fastest runners out first and let everyone else chase. Instead, the slowest runners—the youngest children and the oldest adults—are given a head start, while the fastest ones begin at the back, 25 minutes later. Everyone else starts in waves in between, distributed by a complex formula of age and gender.
The result is a chaotic chorus of “On your left!” as some 1,500 runners pour through the narrow trails of Mount Tam. The system also produces unusual leaderboards. In 2010, an 8-year-old girl won the race, edging out a 68-year-old woman by seven seconds. This year, eight of the top 10 finishers came from different starting groups, and the fastest scratch runner finished 39th overall.
The eldest runner in the race, Mr. Kahn started at 8 a.m., in the first wave, among 6- and 7-year-olds, and other mature runners. Over the next few hours, hundreds passed him.
It was Mr. Kahn’s 21st Dipsea. He ran it nearly every year from age 52 to 75, placing in the top 50 in almost all of them, before taking a 15-year break. Last year, at 90, he returned and set a course record for his age group, breaking a mark previously held by Jack Kirk, the legendary runner known as the Dipsea Demon.
Mr. Kahn admits he is not built like a distance runner. He is compact and solid, with broad shoulders and muscular arms from a lifetime of working with his hands. His specialty was always the downhill. He remembers when he could make up time by bounding down steps and rocks, passing other runners on tight corners like a speedskater.
Those days are gone. His gait has slowed; he takes the hills more gingerly. But his approach to motion has not changed much.
“What he’s always told me is, ‘Just don’t stop moving,’” said his son, Evan Kahn, a shift operator at the Bolinas Community Public Utility District, who joined his father in the race.
The younger Mr. Kahn, 45, had not run the Dipsea since he was 11. After a trampoline accident five years ago left him partially paralyzed, he recovered with the idea of one day running the race with his father. They started 48 minutes apart, and Evan caught him only in the final half-mile. Then they crossed the finish line together.
“Exercise cures every malady,” Lloyd Kahn said. He gave up surfing about two years ago, but he still paddleboards, cycles, runs, hikes the hills and swims in the bay. He picked up skateboarding at 65. A few weeks before the race, he went pig hunting with a group of 30-somethings near Red Bluff.
That creed was reinforced by his years publishing fitness books like Bob Anderson’s “Stretching,” which has sold nearly 4 million copies.
On Sunday, Mr. Kahn’s clock time was 2:02:12, about 12 minutes slower than last year. He had hoped to do better.
But there is always next June. He plans to train harder and smarter, to run the course a few times in the months before the race and to climb the 672 stairs often enough that they do not feel so brutal.
“It’s all about getting used to some pain,” he said. “Get out of your comfort zone, get in the cold water. After you do it, you’re always glad you did.”