Story highlights BethAnn Telford, 47, will run seven marathons in seven days on seven continents

Telford was diagnosed with brain cancer in 2005

Her goal is to raise $1 million to support brain cancer research

(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 am not the one to sit back and watch myself deteriorate."

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