A 33-year-old ultra-endurance athlete has completed an unprecedented swim down the entire coast of California, finishing the 900-mile journey in less than four months.

Catherine Breed will celebrate the end of her swim at San Diego’s Coronado beach on Monday afternoon, 111 days since she began her journey on 1 July at the northern border of California.

“For months, I measured my world in the next few miles,” Breed, a Bay Area native, told the San Francisco Chronicle on Sunday, describing how she had moved through the ocean with “whales beneath me, plastic beside me”.

The record-breaking American open water swimmer previously swam the English Channel in 2018, and completed the North Channel crossing between northern Ireland and Scotland in 2019 – widely considered one of the toughest endurance swims in the world. But those famous 21-mile open ocean crossings were just a quick dip in the pool compared with the 900-mile wild swim along the California coast that Breed and her team began planning three years ago.

“It is like doing a trail 50k or a half Ironman every day for about 110 days,” Breed said in a social media post.

Breed has said she planned the coastal swim to test her own personal endurance, celebrate her love of the California coast, put a spotlight on environmental preservation efforts, and encourage more women and girls to spend time in and on the ocean.

Breed swam the coast in stages, completing about 10 miles per day, accompanied by a shipboard support team who are also officially documenting the endeavor for the World Open Water Swimming Association. Breed slept onboard the ship and made stops along the coast to do events with ocean conservation groups. She also streamed much of her swim on YouTube, including Monday’s “historic finish”.

One of the biggest challenges came at the very end of the swim, Breed said, navigating through the polluted waters by Imperial beach at the US-Mexico border, which she called “some of the dirtiest water in the United States, if not the world”, and that felt like “swimming through sewage”.

“Everyone’s been so fixated on sharks,” Breed said in a video posted on her Instagram Sunday. “We are so scared about these animals that live in the ocean and have no interest in us, and we are more fearful about that than we are about the damage and destruction that we are causing to our own home.”

The swimmer’s unprecedented journey came during what is expected to be a season of unprecedented Super El Niño weather in the Pacific Ocean, with multiple tropical storms, dangerous waves that battered the California coastline, and a massive surge of warm coastal water called a “Kelvin wave” that moved northward as Breed swam south. Breed told local news outlets that the extreme weather has been the hardest part of the challenge, telling San Diego’s KPBS that because of the Super El Niño, “the current is quite literally going against us the entire way”.

As part of the journey, Breed has reportedly navigated waves as high as 7ft, which she described as feeling like swimming “in a washing machine”.

Breed, a former University of California, Berkeley, long-distance swimmer and member of the US national team, transitioned to open water swimming after graduation as part of San Francisco’s Dolphin Club. She grew up in, on and around the water on California’s coast, telling local radio station KQED: “At one point in my life, I think I had swum and sailed under the Golden Gate Bridge more times than I had driven over it.”

In 2017, she set the record for the fastest person to swim the length of California’s Lake Tahoe in just under 9 hours. In 2022, at age 29, Breed became the first person to swim from San Francisco’s Golden Gate Bridge to Half Moon Bay, a 25-mile swim. Last year, she completed a 30-mile swim through shark-filled waters from the Farallon Islands to Golden Gate Bridge.

“You cannot bring hubris or expectations into the ocean – whether you’re surfing, spearfishing, sailing or swimming. You can’t,” Breed told KQED earlier this year. “When I get in the ocean, it’s a full surrender. You’re completely present. Your only job is to put one arm in front of the other and be at peace with whatever the ocean gives you that day.”