He personally took on about $85,000 in loans and credit card debt to underwrite costs of the trip, including the custom construction of his 20-foot boat, Lucille, named after his father’s mother. An additional $45,000 came from sponsors and online fund-raising.

The debt was a source of stress, he said, adding: “I’m a teacher. I don’t have $85,000 sitting in a bank account.”

A cross-country and track athlete in high school, Mr. Carlson said he found his niche when he joined the rowing team in college. From there, he began taking on ultramarathon races, which were critical in training him for the punishing toll of the trans-Atlantic voyage.

A two-time participant in the Spartathlon in Greece, in which runners have 36 hours to run 153 miles (246 kilometers), Mr. Carlson also ran across the United States in a staged race, covering the equivalent of about 30 miles a day.

Those events taught him how to manage the nagging inner voice that can sow doubt and undermine confidence, he said. “Spending any mental energy looking back cannot help you move forward,” he said.

That kind of disciplined thinking allowed him to troubleshoot during various crises on the water.

The first time the boat capsized, he was in its watertight cabin, but an air intake valve was open, allowing water in. The electronics became less reliable and would spontaneously shut off. He had multiple backup navigation systems, but the intermittent failures of the electronics were unnerving.

“Were critical components going to fail and not come back on?” he said.

The boat was built to right itself, “which it did brilliantly” each time, and the first capsizing did the worst damage, he said.