The effort is intended to mark the 250th anniversary of US independence.
The 27-year-old set off on Sunday from the Golden Gate Bridge in San Francisco, California, and plans to complete one marathon per day while travelling between states with a support team in a camper van, including runs in Alaska and Hawaii.
Sobania told Poland's PAP news agency earlier this year that the challenge would be a "logistical nightmare."
If successful, he would become the second person to complete the feat, after American ultramarathon runner Dean Karnazes, who achieved it in 2006 and whom Sobania has described as an inspiration.
He said his goal is to encourage people to become more active and take control of their lives.
"I have a chance to do something that only one person in the world has done," Sobania said. "For me, it’s proof that we can do more than we think is possible."
Sobania has built a following in Poland through a series of endurance challenges.
In March last year, he completed a run across the United States, setting off from New York's Central Park on September 15, 2024 and finishing nearly five months later in San Diego.
He said he completed 125 marathons over 139 days, covering more than 5,250 kilometres, and described himself as the first Pole to achieve such a crossing.
Tomasz Sobania (left). Photo: PAP/Marcin Cholewiński
His previous challenges included a 2023 run from Chorzów in southern Poland to Marathon in Greece and back, a 60-day run in 2022 from the Polish city of Gliwice to Barcelona in Spain, and a run to Rome a year earlier, during which he raised funds for a child with cancer and attended a papal audience.
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Source: PAP