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An American endurance athlete who became the first person to cross Antarctica solo unassisted — a feat some had deemed impossible — said the 54-day trek was as much a mental challenge as physical.

"I would, without a doubt, say that the mental part is harder. The physical part I would say is required," Colin O'Brady, 33, said in an exclusive interview with NBC News.

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"The fortitude to keep going every day through those conditions no matter what was happening. ... At the end of the day the mental is really crucial in achieving this," he said.

O'Brady completed his trip on Dec. 26 by crossing the point he had set as his finish line, the Ross Ice Shelf, the largest ice shelf on the frozen continent. He began the trip on Nov. 3 at the Ronne Ice Shelf, and trekked 930 miles to the end.

He said he cried tears of “joy, elation, exhaustion" when he finished the journey he called "the impossible first" because of people telling him he wouldn't be able to do it.

“That was a really incredibly special moment,” O'Brady said. “To actually be in that moment of finishing and know the impossible has now become possible — was an incredible, incredible moment for me," he said, calling the experience "humbling."

O'Brady, who said he had trained diligently for the trek across Antarctica, had to battle storms, frigid temperatures and wind gusts that sometimes reached 50 miles per hour, all while pulling a sled packed with food, fuel and supplies.

He is now looking forward to being back home in Portland, Oregon, and said he wants to continue sharing his story in hopes of encouraging others to go after their dreams and face any challenges that may arise. He also isn't counting out another journey.

"I want to continue to push the boundaries of human potential. I'm sure there'll be another expedition or project that I kind of dream up," O'Brady said.