A few years ago, when Harper Spain was told she couldn't join her second-grade running club at her school in Wisconsin because she was too young at the time, her mom, Laurel Mullen, had an idea: they'd run together.
"I was like, yeah, let's do this," Harper recalls to PEOPLE exclusively.
The mother-daughter duo jumped headfirst into the sport, running around their neighborhood and nearby areas. Their passion only grew stronger after the family moved to Las Vegas, where they joined the Grace Run Club. Although the club was mostly adults, it became the perfect place to nurture Harper's love for running.
Their early runs quickly led to bigger goals. After completing a 10K, Harper set her sights on a half-marathon at Mount Charleston. When she crushed it, she turned to her mom and said, "I bet I could do a full, Mom."
Now, Harper, 9, and Mullen, 28, are training for that very challenge: their first marathon together.
"It's all about pushing ourselves -- Harper sees what she can do and thinks, I can go further, I can do more, I can push myself even more," Mullen says.
To prepare, Mullen says they created a general training plan and wrote it down on a Taylor Swift calendar. They stick to the schedule with set runs but don't follow strict paces since Harper is still so young. Instead, the focus is on listening to her body -- logging the miles, taking walking breaks when needed, doing strengthening exercises to prevent injury, stretching and staying consistent.
Running has also become a playful bonding ritual, complete with matching outfits and themed race days.
"She's motivated -- she's a motivated girl," Mullen says. "This has all been her idea, and I'm just the support person in the background, buying matching outfits and supplying the snacks on the run."
The two have also developed strategies to push through tough moments. "When we're in the pain cave, we have mantras," Mullen explains. " 'We're strong. How hard could it be if the boys do it?' Little sayings like that get you through the rough patches. The hard work is the training; the race is the celebration."
Music also keeps their miles fun. Harper's playlists often include artists like Swift and Hannah Montana.
"We've got our own little playlist, and it really goes all over the place," the mom adds.
For Mullen, the journey has been life-changing. A mom of three, she was once very overweight and even needed back surgery, which launched her weight loss journey. At the time, she says she hated running.
"I was scared," she admits. "It was embarrassing because I wasn't good at it. So really, we started at the same time. I'd say we're pretty close to the same level of capability. That's been nice -- where she's struggling, I'm struggling just as much."
Their passion has even spread to the rest of the family. Slowly, Harper's brothers have started joining them on runs. Her dad also began running with them when they train on her uncle and aunts' course.
"Except for our full marathon, everyone from Wisconsin is going to fly into Vegas," Mullen says. "We'll stuff an SUV full of everyone and drive to California. The family's going to run the half-marathon while we do the full, and she's kind of inspired everyone in our family to start as well."
"You can make it a family thing," she adds. "It's not just about exercising -- it's about building connection and bonding. Screens and movie nights are fun, but getting outside together, training for a race, even a local Turkey Trot, is special."
Throughout their journey, Mullen began sharing their progress on TikTok and Instagram. What started as fun little videos quickly took off when a clip of Harper running her first half-marathon went viral. "Everyone was commenting, 'This is so inspiring. I love this,' " Mullen says.
Since then, the duo has grown a supportive online community of more than 16,000 followers.
"Every time I tell my friends what place I get in a race, they get really excited," Harper says.
In the week leading up to a race, Mullen says they focus on carb-loading, enjoying plenty of pasta, bread and other favorites. Race mornings are extremely early -- for their last half-marathon, Mullen's alarm went off at 2 a.m. "We end up going to bed around 6:00 p.m. the night before," she says.
Race day itself is an event. Harper and Mullen braid their hair, add glitter and board buses to the start line, often waiting an hour or more in the freezing cold. "These are all downhill races through the Revel Race Series. They're fun and fast, but that also means you have to hang out at the start in the cold," Mullen explains. "Everyone looks like little baked potatoes in their little metal blankets, all huddled around."
Harper remembers one particularly chilly morning: "My hands got totally numb because we forgot to pack our gloves, so I just picked up some on the ground," she says.
Despite the early wake-up and freezing temperatures, Mullen says the atmosphere is energizing. "It kind of evens out because everyone's in the same boat," she says. "The vibes could be low but everyone has so much energy and the mood is just so good. It's so fun."
Even as race day approaches, Harper is already looking ahead.
After this race is over, she hopes to shift her own focus to launching her own youth run club in Las Vegas -- which she's calling Mini Milers -- to give kids a space in a sport that's usually geared toward adults. Her goal is simple: to inspire other children to discover running, whether short or long distance.
"At this point, we're running at least a half-marathon every weekend leading up to race day, which is tough on adults, let alone kids," Mullen says. "So the idea is just one full marathon a year. After that, she wants to try triathlons -- spreading the training across swimming, biking and running -- and [she] just keeps asking herself, 'What's the next thing I can achieve?' "