Together, they constitute the Abbott World Marathon Majors, a collection of the most distinguished marathons in the world. Since 2006, only about 6,500 people have completed all six, the organization said, including Ms. Seman, who finished her final race, the London Marathon, in April.

“She knew that it was incurable from the moment it was diagnosed, and she was determined to make the most of her time,” Ms. Seman’s husband, David Seman, 48, said of her illness last week.

“It almost increased her focus and determination,” he said.

Ms. Seman, who died on Jan. 29 at 42, Mr. Seman said, ran all six races after receiving her diagnosis, drawing the attention and support of runners and cancer survivors. A profile about her in Runners World was published shortly before last year’s London Marathon. In addition to her husband, she is survived by her 6-year-old daughter, Diane. They live on Long Island.

It isn’t uncommon for patients facing a terminal diagnosis to make bucket lists of goals they want to accomplish before they die, said Melissa Ring, the director of regulatory and compliance at the National Hospice and Palliative Care Organization.

“People will take a look back at their life when there is an initial shock of a terminal diagnosis,” said Ms. Ring, who has worked in hospice and palliative care for over 20 years.

“Some people look at how they can get the most out of what they have and how they can get quality life out of that time,” she said. “Sometimes it may be a matter of unfinished things people want to do.”

Ms. Seman started running to become healthy before having her first child, her husband said. She started by running 5Ks and 10Ks, and was training for a half-marathon in Brooklyn when she received her diagnosis, she told Runner’s World.

After learning she had cancer, Mr. Seman said, she thought of only two things: spending as much time as possible with her daughter, and earning the Abbott World Marathon Majors’ Six Star Finishers medal.

“She talked about doing all these marathons around the world,” he said. “She thought, ‘Maybe I could do this.’ She became so focused. It was truly amazing.”

More than a year later, in November 2015, Ms. Seman ran her first marathon, in New York.

“It was something I wished I could do,” she told Runners World. “I just thought, if this was ever something I want to do, I have to do it now. I signed up for it, and I was like ‘What did I just do?!’ I was freaking out.”

Chicago followed in 2016, Boston in 2017 and Berlin in 2018.

In 2019, Ms. Seman ran her last two marathons, in Tokyo and London, eight weeks apart. This required her to train as much as she could while undergoing chemotherapy.

Ms. Seman did not pause or panic. Instead, after Berlin, she began working with Daphne Matalene, 46, a running coach.

“Even when you are super healthy and super trained it still takes a lot out of you,” said Ms. Matalene, who has run five of the six marathons. “Renee was totally undeterred by that. Her goal was not to win; it was not even to run her fastest.”

Ms. Matalene came up with a training regimen that worked around Ms. Seman’s treatment schedule. Ms. Seman would run easy miles in the morning and then have chemotherapy treatment in the afternoon. Days later, once she had recovered, she would do a long run of 12 to 16 miles.

Many runners who try to complete the six races are dealing with health issues or recently had a health scare, said Lorna Campbell, a spokeswoman for the Abbott World Marathon Majors.

“Renee is one of many runners that we have that run the races while facing adversity,” Ms. Campbell said, adding that many people run while dealing with Parkinson’s disease, cancer, depression or other health problems.

“Running is that tool that gives them a distraction,” she added.

Ms. Seman seemed to have razor-sharp focus on her training. Before her final marathon in April, doctors noticed that Ms. Seman was in better spirits when she was able to run.

“She knew she wasn’t going to be here very long, but she still wasn’t going to just give up,” Mr. Seman said, adding, “The cancer overcame her but she was still trying and determined.”

“She wanted her daughter to see someone who was not sick,” he said, “someone who was really tough and did hard things.”