During the week, you likely won’t find retired microbiology professor John B. Hall sitting in front of the TV or teeing up on a manicured golf course. He would rather be outdoors, hiking.

It’s the beauty and solitude of Hawaii’s mountain ridges, valleys and forests that keep him young at heart, and physically and mentally sharp.

“I like the quietness, the peace,” he said. “Hiking, in a way, is a form of meditation, when you’re just doing something rhythmic and you fall into this state where you’re just concentrating on what you’re doing and nothing else matters. It clears your mind of all your worries and problems.”

Hall also enjoys the camaraderie and has met many close friends on the trails.

At 82, he knows practically all of Oahu’s public trails like the back of his hand — he’s spent more than half a century hiking them. He has hiked each at least five times and has a detailed knowledge of them, down to every tree and rock.

“I think you could probably blindfold me and plop me down in the middle of any of the hiking trails on Oahu, and I could probably tell you where I was,” he said.

On average, the octogenarian hikes 4 to 6 miles twice a week — on Sundays with the Hawaiian Trail and Mountain Club, and on Wednesdays with Solemates, a hiking club he co-founded.

The Trail and Mountain Club, founded in 1910, was the only hiking organization he knew of when he moved to the Aloha State in 1962 to become an assistant professor at the University of Hawaii at Manoa.

Hall has served as the club’s president as well as one of its board members and is part of its hard-working, trail-clearing crew that keeps paths free of overgrowth. He still leads some of the club’s hikes from time to time, including, most recently, a newly developed trail around the base of Olomana.

“I don’t try to rock-hop across the streams anymore,” he said. “I cut myself a pole and just wait. My balance isn’t as good as it used to be.”

Fellow HTMC club member Barbara Bruno said Hall inspires her and is one of her favorite people to hike with.

“He knows so much about Hawaii’s plants, and he is always so generous about sharing his knowledge,” she wrote in an email. “Hiking with John helps me see nature through a different lens.”

Hall’s love for botany, an interest he developed while hiking and learning about plants from other club members, eventually resulted in “A Hiker’s Guide to Trailside Plants in Hawai‘i” (Mutual Publishing, 2008), a reference book of plants in five climate zones, with color photos.

When he retired from UH in 1992, Hall was ready to jump into the list of volunteer opportunities he had been keeping in a file. He became a docent for The Nature Conservancy and a mediator for the Neighborhood Justice Center, now known as the Mediation Center of the Pacific.

Today, he occasionally volunteers with the Manoa Cliff native plant restoration crew on Sundays.

Hall has enough lifetime stories to write another book.

Through Fulbright scholarships, Hall, who holds a doctorate in biochemistry from the University of California, Berkeley, trotted around the globe, with teaching stints in New Zealand, Nepal, Turkey and Belize.

He survived a wild rhino attack at Chitwan National Park in Nepal, albeit with seven broken ribs. Others, he later learned, were not as fortunate.

The father of two and grandfather of one has also camped out in the backcountry of Molokai and trekked 11 days through Tsavo National Park in Kenya.

Most memorable of all, he said, was a climb to the summit of Mauna Loa on Hawaii island followed by a hike traversing the Southwest Rift, past two “luas,” or pit craters, with two hiking buddies who had the endurance and courage to attempt it.

“It was something I’d wanted to do for many years,” he said.

The ascent up to the cabin at the summit of Mauna Loa, at 13,250 feet, is itself quite a climb, but making it across the rift while relying on a compass is even more challenging. The trek on a remote and rugged terrain of pahoehoe takes at least three days.

For Hall, retirement is no reason to stop learning. He studies — and teaches — at the Osher Lifelong Learning Institute at UH. He’s taking a Shakespeare comedies class and plans to lead six plant walks this fall, including forays to Lyon Arboretum and the Kaiwi shoreline.

And he plans to keep on hiking for as long as he can.

“You have to find something you like to do or you probably won’t keep it up,” he said. “It makes it easy to keep in shape as long as you’re doing something you enjoy, anyways. So you’ve got to find something you really want to do.”