"Your mental and physical goes up and down," said Nelson.
On Sunday, Nelson will try to become the first woman ever to paddle nearly 40 miles solo from Catalina Island to Dana Point in the annual Ohana Catalina Challenge. Others have finished the race solo but it's only been a handful of men.
Nelson finished the challenge last year as a relay with another woman, switching every 20 minutes.
"The girl that I did it with, she was a nurse, she had to go to the hospital because she was throwing up and had to get hooked up to an IV because she was dehydrated and I came in and thought that I will never do that again," said Nelson.
Nelson is motivated to get back in the race for others. The accomplished surfer wants to raise awareness and cash for breast cancer research. Her mother is a survivor and her close friend is now battling the disease.
"I go with her to chemo a lot and I see her battle and her fight and this is my encouragement to her to just keep fighting," said Nelson.
Nelson has raised nearly $6,000 for two charities: Keep a Breast Foundation and Boarding for Breast Cancer. Her goal is $100,000.
"I think she's amazing," said Ryan Bishop of Dana Point. "I think anybody doing it solo is amazing and then to do it to raise money for breast cancer is outstanding."
During the grueling race, Nelson will also think about her 40-year-old business and training partner, Steve Adler, who passed away suddenly this month from an aneurism.
"I'm going to have a lot of people as motivation for this race," said Nelson.
The top teams usually finish the race in about five hours. Nelson figures it will take her nine.