Bethesda resident Nancy Avitabile trains with free weights for Sunday’s Nation’s Triathlon. (Tracy A. Woodward/THE WASHINGTON POST)

When she wakes up around 6 o’clock on some mornings, and her 64-year-old flesh is telling her not to get up and go running out in the Bethesda cold, Nancy Avitabile always invents a character in her mind. She imagines a San Diego woman, already up and training, in the California heat. That’s the competition, she tells herself. Then she rises and runs for miles and miles in Bethesda’s streets.

All those competitors created in Avitabile’s mind will come alive Sunday, as she and thousands of other athletes from across the country come to Washington for the seventh annual Nation’s Triathlon. The Nation’s course has developed a reputation for being particularly grueling but equally scenic. It includes nearly a mile of swimming in the Potomac River, followed by a 25-mile bike course and a six-mile run that snakes through downtown.

The course is home-field advantage for Avitabile, who has lived in Bethesda for more than 30 years. She has raised her children there. She has started her own accounting firm there. And she has become one of the country’s best female triathletes there — one good enough to qualify for the world triathlon championships next month in Auckland, New Zealand.

“I think people do triathlons for different reasons. Some people just want to cross the finish line, and that’s great,” said Avitabile. “I want to win.”

Avitabile dominated the Nation’s event a year ago, winning the 60-64 age group by 19 minutes. It was perhaps the most emphatic victory of her lifelong racing career, which dates from the 1970s when she was living in Hawaii and was a part of the same Honolulu swim club team that produced Ironman founder John Collins.

But the win at last year’s Nation’s race was less about her finishing time and more about her pain threshold. After winning six triathlons across the country in 2010, her 2011 season had been slowed by Achilles’ tendinitis. Her body kept trying to quit during the race, but her mind invented reasons not to.

“You’ve got to be comfortable with pain,” said Avitabile. “Don’t fear it, embrace it.”

Avitable’s first marathon was 34 years ago, and she began competing in triathlons as the sport exploded in the United States in the 1990s. In the only triathlon she couldn’t finish in her career, Avitabile actually did cross the finish line — in an ambulance. It happened at the Sea Gulls Century triathlon in Salisbury, Md., about 10 years ago, when she crashed on the bicycle course. Avitabile had a concussion and a fractured finger. She’s not only finished every race since but has gradually recorded faster times as she’s gotten older.

Avitabile has already won three events this year, including EagleMan in Cambridge, Md., the New York City Triathlon and the U.S. national championship in Burlington, Vt., the ladder of which she eclipsed last year’s national championship time. In the New York Triathlon’s running leg, she clocked a 51 minutes 40 seconds, which was her fastest time since 2005. At last season’s Nation’s, she registered a personal best on the bike stage, averaging 22 mph on the course.

“She’s kind of an anomaly,” said Susan Hefner, who has been Avitabile’s coach for four years. “As you get older you lose more oxygen. But Nancy has just gotten better and faster.”

The performances are a byproduct of Avitabile treating her body like a temple. She is swimming, running, biking and lifting nearly every day, which works in concert with an intricate diet. She has an army of people monitoring her body every week, including Hefner, multiple trainers and nutritionists, a chiropractor and a masseuse.

“From the moment I met her,”said Geovanny Ardon, who has trained Avitabile for several years at Bethesda Sport and Health, “I realized she had this really strong, kind of tenacious ability to say ‘Hey, I’m going to take this and I’m going to run with it.’ ”

Having mental toughness and using the imagination are two different functions, however — and even though Avitabile will be ready for Auckland come October, she’s slightly disappointed that officials at the world championships haven’t posted a list of the competitors yet. That means Avitabile can only imagine who is part of the world-class field.

“If there’s anyone out there training harder than me,” she said, “then they deserve to win.”