Writer and adventurer Roy M. Wallack has died after a mountain biking accident in the Santa Monica Mountains.

Wallack, 64, was a longtime contributor to The Times, focusing on the outdoors, and the author of sports and fitness books.

He was an avid hiker, runner and cyclist who completed extreme challenges such as the 750-mile Paris-Brest-Paris bike tour and the Badwater Ultramarathon in Death Valley.

Wallack was biking with friends on the Guadalasca Trail, a popular route in Point Mugu State Park, when he crashed while riding down a steep trail and hit his head on a large rock about 9:30 a.m. Saturday, said Capt. Eric Buschow of the Ventura County Sheriff’s Office. He died at the scene.

Wallack’s work for The Times spanned barre classes, triathlons, kayaking, the L.A. Marathon and more. He penned a column on gear for many years.

Last year, he wrote with humor and love about getting a personal trainer for his 89-year-old father.

“My father had no disabilities and comes from a long line of centenarians. To me, the problem was obvious: his Easy Boy chair. Remote in hand, he hadn’t left it in 30 years (except for Costco and cleaning up in the backyard after the dogs),” Wallack wrote.

Wallack’s crusade to keep his father physically active continued, said a friend, Terry Harmon.

Wallack enlisted Harmon to call his father, pretending to be a hired trainer and urging the older man, housebound by COVID-19, to get on the treadmill.

“I’m thinking of the last person he spoke poorly of, and I can’t think of a single person,” Harmon said. “He loved everyone and everything. He was optimistic and encouraging. He was my cheerleader.”

Another friend, Gordon Wright, wrote on Twitter: “One of the greatest characters in my life and a beacon of passion for all things outdoors. We had many adventures together and he died just how he would have wanted to: flying down a mountain on a bike.”

Wallack is survived by his father, Norm; wife, Elsa; brother, Marc; two sisters; and an adult son, Joey.