Never one to waste a spare moment, Matt Fitzgerald clambered into the second row of his Mazda CX-90 on a recent weekday morning and cracked open his MacBook so that he could work on another book.

Mr. Fitzgerald, 52, is many things — writer, public speaker, coach — but mostly he is prolific. He has written or co-written 34 books, most of them about running, endurance sports and nutrition. He writes early. He writes often. He writes a lot.

“Sometimes I do feel like I’m doing B-plus work on a dozen things versus A-plus work on three or four,” he said. “But I am who I am. There’s always a couple of things where I try to give the absolute best of myself at any given time, and I guess that’s enough.”

Mr. Fitzgerald has the sort of slim, athletic build that hints at another part of his identity: distance runner. He has been prolific in that area, too, finishing 50 marathons — his fastest in 2 hours 39 minutes 30 seconds. And, once upon a time, he would have been jogging on the quiet, snow-dusted road in Flagstaff, Ariz., where he had parked his sport-utility vehicle.

Instead, Mr. Fitzgerald was waiting for John Gietzel, a 48-year-old business consultant from Winnipeg, Manitoba, to finish loosening up so that he could close his laptop and coach him through a series of hill sprints. As for himself, Mr. Fitzgerald has barely exercised in three years.

“I probably wouldn’t be doing this if I hadn’t gotten sick,” Mr. Fitzgerald said. “But I’ve found it surprisingly rewarding.”

Mr. Fitzgerald’s bout with long Covid has, in important ways, forced him to reshape who he is and what he does. In the process, he has found vicarious joy by starting a business called Dream Run Camp out of his home in Flagstaff, where he lives with his wife, Nataki, and a rotating cast of recreational runners who pay between $45 and $115 a day to stay in one of four guest bedrooms and be coached by him.

“I’m trying to create a happening,” said Mr. Fitzgerald, who shared his long-term vision: “Fast forward a few years, and everyone in the world has heard of Dream Run Camp, and there’s this mystique about it and it’s all good vibes.”

He organizes group runs every morning. He has “coach’s office hours” every afternoon when he emerges from his writing lair to offer PowerPoint presentations on topics like “Disrupting Complacency” and “Hard Fun.” Mr. Fitzgerald’s campers, whom he calls “dream runners,” can stay for however long they like, up to 12 weeks.

Mr. Gietzel, who has a job that allows him to work remotely, is staying for about a month so that he can train for the Mesa Marathon on Feb. 10. Mr. Fitzgerald plans to be at the finish line.

“There’s some kind of magic here,” Mr. Gietzel said. “I’m already feeling it.”

Mr. Fitzgerald had no way of knowing it at the time, but he now believes that the U.S. Olympic Marathon Trials in February 2020 changed his life. He had traveled to Atlanta to make some promotional appearances ahead of the event and then race in the Publix Atlanta Marathon the day after the trials. “That weekend was much fun,” he said.

After returning home, Mr. Fitzgerald fell ill. His wife soon got sick, too. They both believe they had contracted Covid, though all of this happened before the availability of at-home tests and before widespread government shutdowns.

“We both stayed home and recovered, because hospitals were packed,” Nataki Fitzgerald said.

Mr. Fitzgerald felt horrible for about a month — “It was by far the sickest I’d ever been,” he said — before he slowly resumed his old way of life. In fact, he was running and exercising without issue through the summer of 2020.

“And then it all started to unravel in mysterious ways,” he said. “My neurological symptoms just became showstopping. I couldn’t do anything. I couldn’t write. I couldn’t create a training plan. I didn’t want to interact with people.”

Much remains unknown about long Covid. While there is no test that determines whether symptoms like fatigue, brain fog and persistent headaches are a result of the virus, long Covid can persist for weeks, months or even years, according to the Centers for Disease Control and Prevention.

While Mr. Fitzgerald said his neurological issues had improved in recent months, he still experiences chronic fatigue and “post-exertional malaise,” meaning that anything involving physical effort leaves him feeling awful.

“Exactly the disease you want if you’re an endurance athlete,” he said.

Early last year, he felt good enough to try to ease back into running. After six weeks of gradually building his workload, he was able to jog for 30 minutes.

“And then the bottom dropped out again,” said Mr. Fitzgerald, who has not jogged beyond short distances since.

It has been disorienting for someone whose entire life revolved sports. He recalled one of his fondest experiences as a runner, when he spent 13 weeks training for the 2017 Chicago Marathon as a self-described “fake professional runner” with HOKA NAZ Elite, a Flagstaff-based team of world-class distance runners. Mr. Fitzgerald concluded his time with the team by running a personal-best time for the marathon at age 46, and by writing a book about it called “Running the Dream.”

As Mr. Fitzgerald struggled with the effects of long Covid, he reflected on that experience in Flagstaff. He knew he could no longer run — at least, not anytime soon — but he could envision a way to stay involved, by using his expertise to coach others.

After convincing his wife that they should uproot their lives in California and move to Flagstaff, which is a high-altitude mecca for runners, Mr. Fitzgerald welcomed his first campers — sorry, dream runners — last May. He has hosted about 30 so far.

“I’ve known him to be someone who delivers on his ideas,” said Ben Rosario, the executive director of HOKA NAZ Elite.

Running camps are not exactly a novel concept. Steph Bruce, an elite distance runner, and her husband, Ben, have a weeklong camp for runners in Flagstaff each summer. There are countless others across the country.

The difference with Dream Run Camp is that Mr. Fitzgerald’s dream runners live in his house.

The walls are adorned with artwork of top runners. There is a communal recovery area with a hyperbaric chamber and a contraption called a vibroacoustic therapy bed. His garage is outfitted with high-end fitness equipment. The backyard features a sauna and a small pool for exercise swimming. Mr. Fitzgerald and his wife live in an attached guesthouse.

“It’s a tough thing to promote,” he said. “‘Come to Dream Camp, and be a little bored! It’ll be great for your running!’

“But there’s some truth to it. I see people who come here who are kind of clenched from their normal lives, and after they’ve been here for a few days, they’re liquid.”

While Mr. Fitzgerald seems to have made peace with some of his limitations, he cannot accept being a bystander forever.

Just after midnight on New Year’s Day, he padded downstairs to his computer so that he could sign up for the Javelina Jundred, a 100-kilometer ultramarathon in Fountain Hills, Ariz., in late October. Mr. Fitzgerald acknowledged how incongruous it sounded.

“I literally cannot run one step right now,” he said.

By way of explanation, Mr. Fitzgerald cited Charles Barkley’s final season in the N.B.A. After Mr. Barkley ruptured his quadriceps tendon in an early-season game, he vowed that he would be back.

Sure enough, about four months after sustaining his injury, Mr. Barkley returned to play in one final game, scoring a basket on a putback. He left the court to a standing ovation.

In his own way, Mr. Fitzgerald said, he wants to do the same. He even has a working title for a book that he wants to write: “Dying to Run: An Ailing Athlete’s Quest for One Last Finish Line.”

“I’m not doing this because I’m recovering,” he said. “I’m doing this because I’m not recovering.”

Mr. Fitzgerald does not expect to race, per se. He only wants to finish within the event’s 29-hour cutoff, even if that means walking the course.

“I can just survive,” he said.