ESSEX, Conn. — One of the fastest American marathon runners right now is a 29-year-old doctor in rural Connecticut who doesn’t have a coach, does not have a sponsor and does not run with a watch. He trains alone, as long as feels right, with “only the trees for teammates,” he says. In the past seven years he has raced 47 marathons, winning 19 of them.

“I just go as long as I can before my legs get too heavy and my head gets in a haze,” said Chris Zablocki, the best marathoner most elite marathoners have never heard of.

Last year Zablocki set the world record in the indoor marathon, a 211-lap ordeal around the 200-meter indoor track at the Armory in New York City. He logged a time of 2 hours, 21 minutes, 48 seconds. It was his fifth marathon of the year, and his fifth victory, and it was only March.

Malcolm Richards derailed Zablocki’s bid for a repeat at the Armory on Saturday, clocking in at 2:19:01 to set a new record. Zablocki finished fifth (2:29.13).

Zablocki is the rare athlete who is able to pull off quality races in astounding quantities, emerging as a maverick of the U.S. distance running scene. Zablocki’s strategies are indisputably extreme: He once ran three marathons in three countries in three weekends, winning or medaling in all. And he keeps getting faster: He ran his best marathon ever last December, a 2:13:45. It was the 19th fastest time in the United States in 2017 — and his 11th marathon that year.

“You have guys who run as fast as Chris, sometimes even with full-time jobs like him, but what’s extraordinary is the number of times he does it,” said Jonathan Gault, a staff writer at the leading running website LetsRun.com and Zablocki’s former teammate. “Chris is the kind of competitor who’s always going to give absolutely everything he can, short of dying.”

Zablocki has accomplished this success while on an improvised professional odyssey that has taken him around the world. Since graduating from Dartmouth in 2010, Zablocki has been: a high school teacher on an atoll in the Marshall Islands (where he led students in the first marathon completed by islanders in the nation’s history); a Marine (until he failed to master the requisite synchronized heel pivots and pants blousing and dropped out after a month); a medical student in the Caribbean island St. Maarten (where he enrolled so he could start three weeks after he finished his postbaccalaureate instead of wasting time with an application cycle); and a student doctor doing rotations in Chichester, Britain (where he had his last training partner, who worked at a local condiment factory).

“Chris is unlike anybody else I’ve met; he loves running for the sake of running but also has the ability to push his body to extreme limits,” said the British training partner, James Baker. “He can go through the pain barrier, he has a special ability to recover quickly. And he is genuinely a nice guy.”

Now Zablocki lives at his parents’ home in Essex, Connecticut, while he waits to match for his medical residency next week. He’s hoping for a rural family medicine program in Indiana, which he says he favors for the chance it affords him to drive a tractor and farm kale, his preferred nutritional supplement, in his spare time.

Zablocki’s aptitude for pushing limits began early. He walked onto the cross-country team at Dartmouth his freshman year and finished last on the junior varsity team in its first race. Majoring in history with an emphasis on conflict studies in medieval and Roman eras, he applied his lessons to racing.

“Battles are like a racecourse, with mud spraying everywhere,” he said. “And the only option is to win.”

By his junior year, Zablocki had made varsity, navigating a team dynamic that reveled in taking an endurance approach to running and partying alike.

Allison Benton, who coached Zablocki for eight months in 2015-16 in Britain tried to get him to scale back on his inclination to push every run to its maximum.

“Chris is unique, unlike any other marathon runner I have worked with,” she said. “His ability to recover quickly after a high volume workout or race is exceptional and he uses this ability to do something apparently abnormal in racing so many marathons each year.

“The hardest thing in coaching Chris is to help him to control this urge to push hard on a daily basis.”

After graduating, he set out with some classmates to teach high school on Jaluit Atoll on the Marshall Islands, being assigned history, which came naturally, and chemistry, which did not.

“I told the principal that I got a 43 on my chemistry midterm in college and had to drop it, but the previous chemistry teacher had died the week I got there, and they told me, ‘You’re the best we’ve got.’”

He’d planned to retire from the sport, but a teammate wanted company. Diabetes and obesity were a major problem on the island — the school’s physical education teacher was so overweight he was wheelchair-bound — so Zablocki started taking anyone who was interested along with him on his runs, though they were often running barefoot over the painful crushed coral of the tarmac of the island’s airstrip.

The nation’s Olympic Committee took notice and volunteered to fly three students with Zablocki to the marathon on one of the main islands, provided they proved themselves by finishing a 20-mile training run. They did. Then Zablocki paced them through the marathon in tropical heat: “They raced tough,” he said of them. “One didn’t say anything during the race even though both his big toenails fell off after.”

Returning to the United States, Zablocki spent just over a month in the Marine Corps. He received the most chits of any of his classmates and was punished repeatedly.

“Joining the Marines sounded like an exciting thing to do,” he said. “But it turned out, I couldn’t follow the rules. I was just bad.”

His former teammate Glenn Randall put it another way: “Chris showed up, set the record in the 3-mile run they have to do, but couldn’t march,” he said. “He can’t do the regular thing, but for the hardest thing, he’ll reliably be the best.”

He resigned and pivoted instead to medical school. And despite his looming defense of his world record, he’s most anxious about learning about his professional future next week.

“Honestly, I’m hoping to get a residency and retire from running,” he said. “Although looking back, I’ve tried to retire a few times. It just doesn’t work out.”