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Fred Miller and Shae Brown didn’t meet until last year, but they consider each other family. When Miller’s daughter Alyssa died in 2013, Brown received her heart.

This October, the two will run the Chicago Marathon together—Miller’s 11th 26.2, and Brown’s first.

“When we first met Shae, she said the one thing she really wanted to do was to run the marathon,” Miller, 62, a physician in the Chicago suburb of Glencoe, said. “I said, ‘Well, if you’re serious about it, I’ll join you.'"

"It takes an “incredible person,” he said, to attempt such a challenge after a heart transplant.

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“People who know me that know what I’m doing—they think I’m the bionic woman,” Brown, 49, a dental hygienist in Shiner, Texas, said. “I am so grateful for my second chance at life that I will do everything in my power to stay healthy.”

At age 16, Brown was diagnosed with a cancer called rhabdomyosarcoma. One of her chemotherapy drugs—doxorubicin, dubbed the “red devil” because of its bright color and severe side effects—damaged her heart.

She survived the cancer, but over the years, grew weaker and weaker. The pacemaker she got at age 36 didn’t help much. She’d always loved group fitness classes and other aerobic activities, but as her heart failed, doctors advised her not to push herself.

In 2012, they placed her on the list for a heart transplant. She spent 14 months there, until May 20, 2013—the day Alyssa died, at age 24.

Miller describes his daughter as funny and spunky, more interested in acting and film than athletics but always driven to give back.

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“It was her choice to donate her organs; she had been ill and struggling for a while,” he said.

The family has decided to keep private specifics about her condition.

When Brown woke up after surgery, someone—a nurse, her mother-in-law, she can’t quite recall—asked if there was anything she wanted to do with her new lease on life.

“I said, one of these days I’m going to run a marathon,” she said. “It takes a lot—a strong heart—to be able to run a distance like that.”

Within six months, she began exercising again. As her health improved, so too did her courage. After two years, she found the emotional strength to write her donor’s family a letter of gratitude.

Brown enclosed her handwritten note in a thank-you card. She explained how she’d gotten sick as a child, how the drug that saved her also condemned her. She told them how she hadn’t ridden a bicycle in 20 years without struggling for breath—but that, the October after her transplant, she and her husband biked together on a camping trip.

She was fiercely determined to take care of the tremendous gift she’d gotten, she wrote. She wondered if they’d be open to meeting.

The letter touched Miller, his wife, Barbara, and Alyssa’s twin sister, Eva. They wrote back and said yes, they would see her. So in June 2016, the Millers traveled to Shiner, about 90 miles from Austin, to meet Brown and her husband.

After a day getting to know each other, Eva Miller had a request—did Brown think they could listen to her heart? Brown had been prepared for this. She’d dug up an old stethoscope from hygienist school and brought it out so they could press it to her chest.

“It was a very sobering moment. That’s their baby’s heart beating inside my body,” Brown said. “But a piece of her is still living. Not a day goes by that I’m not thankful for the fact that she wanted to help other people.”

Fred Miller says he thought he’d run his last marathon when he finished Chicago in 2010. He still found tremendous peace in the rhythm of running, but planned to stick to shorter distances.

After hearing Brown’s goal, however, he found himself applying for an entry to the Chicago Marathon. When he got in, Brown cleared the idea with her heart failure specialist and signed up through a charity program called Team One Step. She’s raising funds to send children with cancer to camps much like the one she herself attended as a teenager.

She logs her training miles on the hills in and around Shiner, strategically parking her truck where she can grab her water bottle mid-route. Hydration is essential in Texas summers, where temperatures have routinely hovered in the 90s with near 100 percent humidity.

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Miller’s had mostly milder weather on his runs, which he does largely on trails through a nature preserve in nearby Skokie. He hadn’t realized it, he says, but he’d stopped listening to music when Alyssa died. He picked up his headphones on a recent long run and was instantly transported back in time by his old playlists.

“Losing a child is just a devastating thing—the immediate sense of loss and longing and yearning,” he said. “You never get over it, but you start to just kind of move on, and that’s actually painful. What I hope to do is to make sure nobody forgets her.”

The two runners text and email multiple times per week, sharing training tips, jokes, and inspiration. When race weekend comes, Brown and her husband will stay with Fred and Barbara, and Eva plans to fly in from Los Angeles to cheer them on. Running with their support on the streets of Alyssa Miller’s hometown will honor her memory and also put power in Brown’s steps, she believes.

“I think it’ll still be exhausting, but it’ll make it easier because I won’t be in it alone," she said.

The small-town Texan has watched videos of the race online and can’t even imagine the sheer volume of people and runners she’ll encounter. She hopes to run a little more than 5 hours and has no idea what she’ll do when she crosses the finish line—she may laugh, she may cry.

“I’ve pictured several different scenarios; we’ll just have to see when it happens,” she said.

Despite his 10 previous marathons, Miller isn’t sure what he’ll experience this time, either.

“Finishing—I hope that there’s a feeling that, at least for a moment, we’re together again,” he said.

And though he plans to run beside Brown the entire 26.2 miles, when they reach the finish line, he’ll let her cross first and enjoy her moment: “That’s what dads do.”

This article first appeared on Runner's World.