During the first marathon, Bret Parker felt great — for the first 15 miles of ice and snow.

“I was chugging along, and I had no symptoms,” he recalled the next day. “I was running a good pace. I said, ‘You got this.’ ”

He paused. “And that was the kiss of death. I started slowing down. It got colder. It got windier.”

It was Jan. 30, and Bret was running a marathon on Antarctica. It wasn’t actually that cold for most of the race — about 20 degrees. But it was windy. And Bret has Parkinson’s disease. Like the 50 or so others on this adventure, he wore ski goggles and trail shoes and lots of layers. Unlike them, he carried a tiny plastic bag of pills that he was regularly popping to keep the stiffness, cramping and tremors of Parkinson’s at bay.

The symptoms came anyway. The route on Antarctica, six laps around a four-mile loop at a Russian research station called Novolazarevskaya, featured endless vistas of blue ice all around — like Caribbean waters, only frozen. He walked a lot over the final 10 miles. But with a quarter-mile to go, he started running again. He could feel a symptom coming on that he had experienced only rarely since he was diagnosed with Parkinson’s 11 years ago, at age 38: an uncontrollable, head-to-toe shivering. He knew he had to get inside.

When he crossed the finish line in just under 6 hours, 23 minutes, Bret’s hands had curled up, his calves were cramping and he struggled to speak. He stopped to record a video to post online for the friends and family members following his progress. But he was in such bad shape that the event organizers put him on a snow­mobile for the ride back to shelter.

“It took me a while to finally settle down,” he recalled. “Then we got back on the plane.”

Less than eight hours later, Bret lined up at another starting line along the southern tip of Africa, ready to do it all over again.

A crazy challenge

Bret was competing in the World Marathon Challenge, in which athletes run seven marathons on seven continents in seven days, doing little other than sleeping and eating on a chartered airplane between races.

Since he was diagnosed with Parkinson’s, Bret has been on the hunt for personal challenges. There was his hometown New York City Marathon. There was a triathlon along Long Island Sound. There was a jump out of an airplane. Now, there was this.

“Parkinson’s has given me the freedom, the liberty to take on these things, even though they seem ridiculous,” Bret, executive director of the New York City Bar Association, said in an interview in November. “It helps me get over a fear of water, it helps me raise money, it gives me a goal. It’s a lot better story when I’m trying to tell people to donate to be able to say I’m doing my part.

“Also,” he added, “I don’t want it to be in charge of me.”

“That’s the real issue,” his wife, Katharine, chimed in.

Bret’s Parkinson’s is often hard to notice. His right hand trembles occasionally, and he often exhibits a roly-poly movement in his arms and shoulders that he said is a side effect of his medication. He knew his symptoms would worsen over the course of the World Marathon Challenge, and they did.

Day 2 brought Bret to a beachfront promenade in Cape Town, South Africa. He’d gotten two hours of sleep on the flight from Antarctica. His feet hurt from the first run, and he had decided to walk this race’s four six-mile loops along the Atlantic Ocean.

Yet he sounded optimistic as he prepared to start.

“One marathon down, 6 to go!” Bret posted on Facebook that day. “Antarctica was wonderful and awful! A little banged up (combination of Parkinson’s and running 26.2), but next one is in Cape Town, less than 12 hours after Antarctica and I’ll be on the starting line.”

Embracing a new identity

Bret and I met in 1986, when we lived on the same dormitory hallway during our freshman year at the University of Pennsylvania. We worked on the student newspaper together, but after graduation, he went off to law school and I to my first newspaper job, and we fell out of touch, reconnecting only when Facebook came along.

I remember when Bret revealed his diagnosis on Facebook, in 2012. He’d been living with a secret that he had shared only with Katharine, his parents and a close friend.

Bret and Katharine had hidden his diagnosis — even from their two sons, Ben, now 17, and Matt, 20 — out of uncertainty over how it would change their lives, his career, his ability to be a husband and father, how his friends and peers would perceive him. Katharine recalled wondering whether he would be able to drive, work or even walk.

When he finally went public about his disease, he embraced his new identity. He began raising money for the Michael J. Fox Foundation, whose famous founder, like Bret, had been stricken with Parkinson’s at a young age.

Parkinson’s patients don’t see themselves as terminally ill, but their life spans are typically reduced and there is not yet a cure for the neurological disease, which causes reduced dopamine levels in the brain and a variety of symptoms, including slowed movement, stiffness, cramping and tremors, which typically worsen with age. The average age at diagnosis is about 60, but young-onset Parkinson’s occurs in as many as 10 percent of cases.

Much of Bret’s advocacy has come on social media, where he has built a support network that spans continents and includes people he’s never met. There, questions about the wisdom of taking on bigger and bigger challenges intermingle with straight-up cheers. Both attract attention and donations. Both seem to have the same, motivating effect on Bret.

Running seven marathons in seven days can be damaging to a perfectly healthy adult. But having Parkinson’s doesn’t necessarily make it more so, said Melissa J. Nirenberg, a neurologist and researcher who treated Bret for the first 10 years after his diagnosis.

Nirenberg, who is now the chief medical officer of the New York Stem Cell Foundation Research Institute, said she had the same reaction as many of Bret’s friends when he told her of his plans — “What in the world are you thinking?” I had that reaction too, when Bret posted news of his next challenge on Facebook early in 2017. But Nirenberg noted that exercise is known to slow the progression of the disease’s symptoms.

“There was a time when this would have been an end to a road,” Bret said to me during a call from the Cape Town race route, as he walked swiftly past playgrounds and mini-golf courses in the shadow of Table Mountain. “Right now I feel perfectly fine. I’m very aware of the ultimate goal of running all seven. I just want to get on the plane to Perth tonight and get a good night’s sleep.”

He crossed the finish line in Cape Town at 6 hours, 47 minutes and 50 seconds.

The challenge of Perth

Bret’s condition took a turn for the worse in Perth, Australia. He had managed about six Ambien-induced hours of sleep on the challenge’s chartered plane, which came equipped with lie-flat seats and couches. Among this year’s 50 competitors, Bret was part of a smaller group of 16 who collectively raised about $1 million for multiple charities and whose entry fees, about $45,000 apiece, had been paid by a single, anonymous benefactor.

When Bret woke up as the plane approached Perth, the top of his right foot and his left shin hurt badly. He also had developed blisters, including one on the ball of his left foot that was the size of a large cookie.

On a bus to the hotel where runners would have about 30 minutes to get ready for the race, he sat next to a window and wept.

“I had read all the Facebook comments — ‘You got this,’ ‘You’re awesome,’ ‘You’re a hero,’ ” Bret told me by phone after that day’s race, his voice low and subdued. “And now I’m going to have to tell them, ‘I can’t do this.’ ”

His voice cracked as he continued, describing the Facebook post he had sent out about a half-hour before the start. “It’s possible I won’t make it through this one,” he had written. “I know I shouldn’t even admit that before I start and it’s all very emotional for me, but after the joy I felt (and posted about) I thought I should share the lows as well to keep it real.”

The post generated nearly 300 likes and more than 100 comments. “Bret, you’ve already won,” one friend wrote.

After a few miles in Perth, Bret realized that he could make it if he was careful and persistent and kept to a consistent walking pace. He even began running toward the end of the course, out of fear that he might miss what he thought was an eight-hour cutoff. Running, oddly, was less painful for his feet. He crossed the finish line with eight minutes to spare.

Pushing on

Bret mostly walked the next three marathons — in Dubai, Lisbon and Cartagena, Colombia — but he also broke into some unexpected bursts of running. In the early morning in Dubai, he saw late-night clubgoers on their way home — and heard Muslim prayers coming from the loudspeakers of mosques. In Lisbon, a cobblestone marathon route and cold, rainy weather created problems for all the runners, and Bret walked his slowest race so far, finishing just under 9 hours and 17 minutes. The eight-hour cutoff, it turns out, was not enforced so long as the group could remain on the air charter’s schedule.

But Bret’s physical condition continued to worsen. Toward the end of each flight, he made his way to the couches at the back of the plane, where a race staffer wrapped and taped his left foot, which was covered in blisters and peeled skin. As they flew to the final marathon, the staffer came to him.

When he arrived in Miami on Feb. 5, he could barely walk off the plane. Tumbling out of the bus at the starting line along Ocean Drive in Miami Beach, he found dozens of friends, family and even a few strangers who had learned of his effort through the Fox Foundation and gathered to watch the final race. Katharine had bought orange T-shirts for everyone, emblazoned with her husband’s name. Bret’s father, James, stood along the route the entire day.

Bret was feeling euphoric that the end was in sight. Around 2:20 p.m., he began running. After the first 5.2-mile loop, up and back along the South Beach boardwalk, though, he slowed to a walk. It had been a long seven days.

His son Ben walked three five-mile loops with his dad, at one point carrying a box with a slice of pizza for him. Others joined the procession. When Bret asked for something with caffeine, a lawyer friend who had flown down from New Jersey dashed off to a beachside bar and ran back with a sloshing cup of Diet Coke.

As the day turned to night and footlights came on along the boardwalk, the crowd surrounding Bret grew to nearly a dozen. Along one stretch, he stopped cold and the group froze. A foot was cramping. He resumed walking, his right hand hanging clenched at his side.

It was dark when Bret started the final lap. Katharine joined him for a few minutes, but then he set off on his own.

The crowd at the finish line grew quiet as Bret disappeared from view. Katharine prepared a bottle of champagne. Organizers stretched victory tape across the path.

Right around 10 p.m., Bret’s bobbing white running hat popped into view, with Ben, who had joined him for the final quarter-mile, at his side. Everyone else had finished the race long ago. A deafening chorus of cheers erupted as Bret crossed the finish line. It had been 7 hours, 41 minutes, 22 seconds.

He flashed seven fingers as he broke through the tape. He spun around, smiling, his hands on his waist. He laughed as Katharine popped the champagne, gave her a hug and took the beer she held out in her hand.