Telford’s joy in the accomplishment was tempered by the pain behind her left eye. The brain tumor she has lived with for eight years hurt more than usual. It had been acting up since she finished the Ironman race Oct. 13, and the cabin pressure on the flight home from Hawaii hadn’t helped. Sometimes the tumor caused wave after wave of fatigue, slurring and memory loss.

But none of that will be enough to stop her from competing in Sunday’s 37th Marine Corps Marathon. It will be an emotional day for many of the race’s participants, some of them U.S. military veterans wounded overseas. It will be Telford’s 11th consecutive Marine Corps Marathon, a race that took on new meaning in 2004.

That year, as she cleared Hains Point, between mile marker 18 and 19 on the course, she felt a pop in her head. Something detonated, and Telford’s vision went blurry. When she looked up at the sky to clear her head, the only image that sticks in her mind was a light post that was set alongside the path.

“I wiggled my head around, almost like it was a severe headache, where you start seeing stars. I saw that light post,” she said.

She finished the race, but barely. In the days that followed, she began to run into furniture in her Fairfax home because she forgot where it was. Then seizures set in. Doctors soon found a large tumor behind Telford’s left frontal lobe, an egg-shaped growth wrapped around a major blood vessel in the brain.

“She was in her 30s at the time. You don’t think something this catastrophic is going to happen . . . especially to somebody that is young and fit,” said Bob Latin, who has been Telford’s partner for 11 years. “I was stunned.”

Over the past eight years, Telford has endured two major brain surgeries at Johns Hopkins Hospital to remove the growth and regrowth, and she is scheduled for a third early next year. The cancer has spread to her bladder, bringing on several more surgeries, and now she fears that she has a tumor near her larynx, which she said was discovered a few weeks ago before she left for Hawaii.

Telford has learned how to walk again. She has learned how to see again, after losing the vision in her left eye. She has learned how to speak differently, because sometimes the cancer causes her to stutter and slur her words.

But she has run in at least one marathon every year since her 2005 diagnosis. Telford has completed the Boston Marathon in each of the past three years, and this month’s Ironman was her second in four years.