As Cara Linehan Buckwell prepared for the Boston Marathon in 2011, her thoughts were on more than the many miles from start to finish. She was running to raise awareness and research funds to spare families what hers endured when her youngest sister, Laura Linehan, died at 20 after the wait for a liver transplant stretched too long.

“For me, it’s just a great way to honor Laura’s memory, to give back and keep her memory alive,’’ Cara told an interviewer.

Six years later, and just months after becoming a mother, Cara was diagnosed with lung cancer — a mistake of nature, her doctors said.

“She treated her body as a temple for years, eating the right foods and exercising,’’ said her mother, Ann McCarthy Linehan. “She followed all the rules, and I can’t believe this happened. Of all people, it shouldn’t have happened to Cara.’’

She died Tuesday at 34, a couple of years after moving back to the Melrose neighborhood where she grew up. She was a Division I swimmer in college, a respected coach at her high school alma mater, and ran races to boost her fund-raising total. When she died, her daughter, Claire Christine Buckwell, was 10 months old.

“As a family, we lived through this before,’’ said her father, Jim Linehan. “We thought that having gone through this once, it was impossible to go through it again. The odds would just not allow.’’

Determined as anyone could be to transcend impossible odds, Cara looked toward the future until the end. After being diagnosed in March, “she literally didn’t catch a break the entire time,’’ said her husband, Dan Buckwell.

Yet throughout “she barely cried. She was so focused, and after every treatment she got bad news,’’ said her sister, Drew Jacobs. “And after she heard the bad news, she’d say, ‘OK, what’s next?’ ’’ As doctors offered new treatments, “she would always say, will it keep me around to be with my baby?’ They said yes, and she said, ‘Let’s do it.’ ’’

Because of Laura, who needed her first liver transplant at age 2 and died in 2008, “needless to say, Cara was an organ donor,’’ Dan said. Her illness ruled out certain organs, but Cara donated her corneas so someone might regain sight, and donated tissue samples for cancer research.

“With any luck, once they’re studied, we can help prevent other families from going through what we’ve gone through now,’’ Dan said. “If any good can come of this, she’s already made sure of it.’’

Cara A. Linehan was the oldest of Ann and Jim’s three daughters and grew up in the Melrose neighborhood where her parents, husband, daughter, and sister all live.

“She had the same qualities as a child that she did as an adult,’’ Ann said. “She was an amazing woman. She was extremely strong, determined, motivated. Cara taught us how to be parents. That’s my husband’s line, I shouldn’t be stealing it, but she was.’’

Cara, Jim added, “was always taking care of her little sisters. It was fun to watch her, but she was the boss of them in a sense. When her mom wasn’t around, she knew what to do.’’

She began swimming competitively in fifth grade, and after graduating from Bishop Fenwick High School in Peabody she was on the swim team at the University of Maryland, Baltimore County. She majored in graphic design, with an art history minor, and went to work as an art director at Vistaprint, most recently in Waltham.

“Cara was also a lot of fun,’’ Drew said. “She had so many friends. She had way more friends than I did, and they all adored her. And she didn’t have just one best friend, she had tons of best friends.’’

Those overlapping circles of friends — from high school and college, from swimming and work, from Melrose and Boston — brought her together with Daniel Buckwell, who was from Hamilton and whom she met when they were freshmen at different colleges. “We first shook hands and said, ‘Hey, how are you,’ in 2001 but didn’t go on our first date until 2007,’’ he recalled.

They were back in the Boston area by then and had been running into each other for years. They married in 2012. “People always say you should marry your best friend, and don’t worry about all the superficial things. At this point I couldn’t agree more,’’ he said. “She truly was my best friend, and every day she was amazing.’’

Known for her compassion, Cara was a mentor to newcomers at work and at Bishop Fenwick, where in 2016 the Globe honored her as the Division 2 girls’ swimming coach of the year, after she guided her team to repeated undefeated seasons.

“That was her way of giving back, of being able to affect somebody else’s life in a positive way,’’ her father said. “She would get kids in the water, teach them the strokes, and show them what a team was all about. She had some kind of magic in getting them to trust her. You could see confidence build in the kids.’’

He added that “between her coaching and her art, she lived a good life and was hugely successful in my eyes. I couldn’t have been more proud.’’

When Cara was pregnant with Claire “she was all about doing things the right way,’’ her mother said. “She so badly wanted a family. She had so much love in her.’’

During the years her sister was ill, Cara made meals that accommodated Laura’s restricted diet, including salt-free french fries. As an expectant mother, she was as precise with her own health. Even when tired on hot summer days, “she would go out and walk two miles, just to make sure she was getting enough exercise,’’ Drew recalled. “Out of everybody in the family, she was the healthiest. She never smoked — not one puff.’’

When Claire developed an allergy in her early months, Cara set aside many of her own favorite foods, including cheese pizza, while nursing. “She always said that after Claire grew out of her allergy, every Mother’s Day she was going to make Claire buy her a cheese pizza,’’ Drew said.

In addition to her husband, daughter, parents, and sister, Cara leaves her grandmother, Elizabeth Linehan of Cambridge.

A funeral Mass will be said at 10 a.m. Monday in St. Mary’s Church on Herbert Street in Melrose.

“She lived for Claire and wanted nothing more than to see her grow up,’’ Dan said, adding that while Cara was alive, “Claire started crawling and learned to roll over. It may sound silly, but those little accomplishments made Cara so happy.’’

Through the example she set as a mother, an older sister, a coach, and a friend, “Cara accomplished something I think we all aspire to do, and that’s to leave the world a better place than the way she found it,’’ Dan said. “She’s had such a big impact on Melrose and Boston and New England and further. She’s reached people everywhere. I can only hope in my life to do nearly as much good as she did in the 34 years that she had.’’

Bryan Marquard can be reached at bryan.marquard@globe.com.