(CNN) For the past three years, BethAnn Telford has trained for a grueling athletic achievement, the World Marathon Challenge. It's seven marathons on seven continents over seven days. An ambitious goal, but not her biggest challenge.
"I am the only USA woman and the only person with live cancer cells in my body [competing]," she said.
Long before Telford's cancer diagnosis, she was an avid marathoner. In October 2004, as she headed toward the Marine Corps Marathon finish line, she heard a loud pop in her head. Worried but committed, she ran the remaining 7 miles to complete the race.
The next Monday, BethAnn felt peculiar. She grew distracted. She couldn't remember dates or names. When she failed to locate the bathroom at work, her boss recommended some time off.
That's when an MRI revealed a brain tumor.
"I was stunned," said Telford, 47. "Twelve years ago I never heard of a brain tumor. I didn't know that kind of thing could grow in my head."
Telford concealed her diagnosis for almost half a year. "I didn't want to devastate my family because I didn't want them to know that I might not be around much longer," she said.
The next year, surgeons removed most of the tumor, but one step forward introduced a whole new set of complications.
"I found out I could not have children. I lost sight in my left eye. I had to learn how to stand and walk again. I got seizures. My bladder shut down. I will have to self-catheter for the rest of my life," she said.
None of that stopped her.
"I am not the one to sit back and watch myself deteriorate."
Five months later, Telford started running again. Her first marathon after the surgery was the same race where she'd experienced brain cancer symptoms a year earlier. She's run in it every year since.
Dr. Henry Brem, chief of neurosurgery at Johns Hopkins is one of Telford's physicians. He said he is astonished by her dedication, so much so that he is enlivened to find new treatments.
"Sometimes we as neurosurgeons who are on the front lines do it because we're motivated to help people and we're trained to do it," said Brem. "But we take an amazing amount of inspiration from patients like BethAnn."
Throughout the World Marathon Challenge, Telford will race in Antarctica, Chile, the United States, Spain, Morocco, Dubai and Australia.
Telford partnered with a nonprofit devoted to raising funds for brain cancer research.
Telford already raised more than $800,000. The goal is to reach $1 million.
Along her journey, she'll run in shoes plastered with pictures of children battling brain cancer. She said it's an homage to all children with the disease, specifically the ones who cannot afford treatment.
She choked up at the thought of it. "We need this funding for these kids," she said.
As for whether Telford will be able to complete the races, she said, "My finish line isn't sitting at the end of the line of this big, epic event. My finish line is when someone announces there is a cure."