When Joan Ullyot, a physician and an accomplished runner, published her book “Women’s Running” in 1976, she took on a daunting set of traditional ideas that boiled down to one admonition: Women should not run long distances.

They were not physiologically built for it, women were told. Compared with men, they typically had higher body fat, less muscle bulk and lighter bone structure, factors that should discourage them from engaging in long-distance running — or so it was believed. Moreover, many authorities in the field warned that extended running might harm women’s reproductive organs.

But Dr. Ullyot (pronounced UH-lee-yet) methodically debunked those assertions in her book, one of the first to examine the sport from a female perspective, and one of the first books on the subject by a female author.

“You just have no idea how many myths and superstitions there were around vigorous activity for women then,” the marathoner Katherine Switzer said, “so we needed this a lot.”

For Dr. Ullyot, “the purpose of the book was to tell women what they could do in running, not what they couldn’t do,” she was quoted as saying in the book “First Ladies of Running” (2016), written by the marathoner Amby Burfoot.

Dr. Ullyot, who was 80 when she died of cardiac arrest on June 19 in Palo Alto, Calif., was an accomplished marathoner herself, and in the 1980s she was instrumental in successfully pressing the International Olympic Commission to include a women’s marathon in the Games. (With few exceptions, the Olympics had a long history of barring any kind of long-distance race for women until 1960.)

As a member of the International Runners’ Committee, an advocacy group formed in 1979 to lobby for the inclusion of women’s long-distance races in international competition, Dr. Ullyot used her research in presentations that the group made to the I.O.C. in the run-up to the 1984 Summer Olympics in Los Angeles.

In making its case, the group was up against a history of open hostility to female competitors in marathon races. Ms. Switzer famously entered the 1967 Boston Marathon under the name K.V. Switzer to pass as a man. When race officials got wind of a female entrant, they attempted, without success, to physically remove her from the course. She was the first woman to complete the race as an official entrant.

In 1977, the I.O.C. refused to add a 3,000-meter race for women at the 1980 Games in Moscow, though an article in The New York Times that year cited Dr. Ullyot’s research as demonstrating that women “may be physiologically able to keep on running and running — and running.”

The I.O.C. ultimately relented, and both the 3,000-meter race and the marathon for women were added to the 1984 Games, in large part because of the efforts of Dr. Ullyot’s group.

Joan Benoit, an American, won the first gold medal in the 1984 marathon event, with a time of 2:24:52. The Times’s coverage of the race bore the headline “Women Athletes Topple Sports Myth.”

Joan Wingate Lamb was born on July 1, 1940, in Chicago to Theodore and Deborah (Bent) Lamb. Her father was an architect, her mother a homemaker. Joan, her parents and a sister lived on the Upper East Side of Manhattan and moved to California when Joan was in high school.

She was not an athlete as a child; as she told Gary Cohen, a running blogger, in a 2017 interview: “Girls didn’t run. I don’t know why, but they just didn’t.”

Dr. Ullyot attended the Westridge School, a private school for girls in Pasadena, Calif., then went east to Wellesley College in Massachusetts, graduating in 1961 with a degree in German literature. She attended Harvard Medical School and graduated in 1966.

After marrying Dr. Daniel Ullyot, a heart surgeon, in 1965 and moving to San Francisco four years later, Dr. Ullyot began to run as a way to lose weight. She took to it immediately and entered her first major race, the 12-kilometer Bay to Breakers in San Francisco, in 1971, the first year the event was open to women.

Dr. Ullyot ran more than 75 marathons and numerous other races while working as a medical researcher in cellular pathology at facilities like the University of California, San Francisco Medical Center.

She was a single mother of two sons for much of her life: Her marriage ended in divorce in 1976.

Her son Theodore Ullyot recalled that she had mandated that he and his brother, John, run three miles to middle school with her a few days a week, backpacks and all.

“It was extremely embarrassing for a couple of teenagers,” Mr. Ullyot said. Nevertheless, both sons took up running as well.

Dr. Ullyot, one of six runners from the United States, attended the first international women’s marathon, in Waldniel, Germany, in 1974. Having learned German in college, she served as the team’s interpreter.

During the event she met Ernst van Aaken, a German doctor who was an early proponent of women’s running. He pioneered the “long slow distance” method of training, which emphasizes running long distances at a slow speed, rather than the shorter-distance and more intense interval training that was standard at the time. The two became friends and collaborators.

Together they developed training programs, for both men and women, to maximize endurance and minimize damage to the body. (She told Mr. Cohen, the running blogger, that Dr. van Aaken, who died in 1984, had introduced her to one of her favorite non-running activities: drinking wine.)

Dr. Ullyot was a fixture in the starting blocks of the Boston Marathon, running nine of the races and winning the Masters division for runners over the age of 40 in 1984.

In 1990 she married Charles E. Becker, who was also a doctor. The couple moved to Snowmass, Colo., and Dr. Ullyot coached a running club in Aspen.

In addition to her son Theodore, she is survived by her husband; her son John, who confirmed her death; her sister, Deborah McCurdy; two stepchildren; and six step-grandchildren.

In addition to “Women’s Running,” Dr. Ullyot’s books include “Running Free: A Book for Women Runners and their Friends” (1980). She also wrote a column for Runner’s World magazine.

Dr. Ullyot ran her last marathon, the Boston, at 56. But she never lost her competitive spirit. When her son Theodore ran a marathon in two hours and 50 minutes, beating Dr. Ullyot’s personal record, she resolved to outdo him. After intense training, she beat his time by a full two minutes.

Mr. Ullyot recalled his mother saying, “You boys can have all the records in this family short of a marathon, but you’re not taking the marathon record from me.”

“I’ve had a motto since turning 40,” Dr. Ullyot told The Times in 1989, when she was 48. “Age, experience and cunning can overcome youth and ability.”