Grete Waitz, whose nine New York City Marathon victories made her one of the dominant runners of the 1980s and helped destroy the notion that women were too delicate to race such long distances, died April 19 of cancer at a hospital in Oslo. She was 57.

The Norwegian-born Mrs. Waitz was a graceful and nearly invincible force from 1978 until 1988. She won five world cross-country championships and set scads of world records, including in the 3,000-meter, 10-kilometer, 15-kilometer, 20-kilometer and 10-mile.

She was most closely identified with the 26.2-mile marathon — especially the five-borough New York marathon, where she broke the world record three times and became a crowd darling.

She was the first woman to run a marathon in less than two and a half hours, a feat previously thought to be impossible. Her strength and speed inspired a generation of amateur runners and helped prompt International Olympic Committee to allow women to compete in the Olympics’ 3,000-meter and marathon events for the first time in 1984.

“Grete Waitz is a history-maker, one of the truly epochal athletes in women’s sports,” journalist Peter Gambaccini wrote in Runners World in 1994.

Mrs. Waitz discovered marathoning by accident. A full-time schoolteacher in Oslo who trained before and after work, she spent the first decade of her career competing in short- and middle-distance track events.

In the 1970s, she broke the 3,000-meter world record twice. She was a two-time Olympian in the metric mile, then the longest Olympic race open to women. But by the time she turned 25, she felt her career had stagnated. Her Norwegian teammates, all younger, nicknamed her “Grandma.”

She was on the verge of retiring from the sport when, at the urging of her husband, Jack Waitz, she accepted an invitation to compete in the 1978 New York City Marathon. Race director Fred Lebow chose her as a “rabbit,” charged with setting a strong pace early on for serious competitors. She had never run farther than 12 miles at one time, she later said.

Mrs. Waitz and her husband, who was her coach, treated the trip as a second honeymoon. The night before the marathon, they ate a four-course meal of shrimp cocktail, filet mignon, red wine and ice cream.

The next day, Mrs. Waitz set a blistering pace. She kept it up through all 26 miles and 385 yards, enduring dehydration, cramps and a level of screaming pain with which she had previously been unacquainted.

“I’ll never, never do this again!” she yelled at her husband as she crossed the finish line, blonde pigtails swaying.

But she had won the race in two hours and 32 minutes, shattering the world record by more than two minutes and inaugurating a new career as a marathoner and an international star.

“To be suddenly a hero on a world basis was hard for me to understand,” she later said. “God gave me a gift. I got the chance to use it. I felt uncomfortable with the credit.”

Mrs. Waitz quit teaching in 1979 to focus on running. She ultimately improved her marathon time to a personal best of 2:24. (The current women’s record is 2:15.) For several years, she won every marathon she finished — including the 1983 world championships at Helsinki.

Then, in 1984, she faced American Joan Benoit in the first-ever women’s Olympic marathon. Benoit won that race in Los Angeles and Mrs. Waitz took silver. Her effusive praise for Benoit and her refusal to make excuses for the loss, despite having suffered debilitating back spasms in the days before the race, earned Mrs. Waitz a reputation for graciousness.

Knee pain forced Mrs. Waitz to drop out of the marathon in her fourth and final Olympics, the 1988 Seoul games. She won her ninth New York City Marathon later that year, making her the winningest runner in the race’s history.

By the early 1990s, Mrs. Waitz had been forced into retirement by an accumulation of running-related injuries.

She continued to attend the New York run every year as a spectator and in 1992 ran alongside Lebow, her friend and former race director, who had brain cancer. They ran slowly together, crossing the line after an emotional five hours and 32 minutes. Lebow died two years later.

Grete Andersen was born Oct. 1, 1953, in Oslo, where her father was a pharmacist and her mother worked at a grocery store. Her parents hoped Grete would become a pianist, but she was bent on emulating her athletic older brothers.

Besides her brothers, Jan and Arild, survivors include her husband.

Her neighbor, Olympic javelin champion Terje Pedersen, encouraged her interest in running, and by 16 she was Norway’s junior 400- and 800-meter champion. She competed in the Olympics for the first time in 1972, at 18.

Over the past two decades, Mrs. Waitz had been one of long-distance running’s most visible and venerated ambassadors. Keeping homes in Norway and in Gainesville, Fla., she traveled widely to encourage runners — and especially women runners — at road races.

She wrote several books about running, chronicling her own career and offering advice.

“I prefer to train in the dark, cold winter months when it takes a stern attitude to get out of bed before dawn and head out the door to below-freezing weather conditions,” she once told an interviewer. “Anyone can run on a nice, warm, brisk day.”