When Shelby Mills was in high school, she sort of backed into running.

“I just kind of figured I wasn’t good at anything else so I started running,” she said.

Once she got going, she couldn’t stop.

Now a 23-year-old who competed in college at Gonzaga, Mills hit a milestone Sunday morning at the 2017 Alaska Airlines Rock ‘n’ Roll Seattle Marathon. Not only did the former 5K and steeplechase runner compete in her first marathon, she won it – on her birthday – finishing in a time of 2 hours, 45 minutes and 38 seconds.

“It’s my first one,” she said. ”I ran in college, so I just graduated in 2016. I’ve been running a lot and I figured, before I put my mileage back down I’d do a marathon while I was still up there.”

Mills grew up in Snohomish, WA, she lives in Seattle’s Queen Anne neighborhood and the medical assistant runs almost 100 miles a week.

“It’s an OCD thing,” she said. “It’s like a runner’s high at all times.”

But, while she figured her birthday was a good time to run her first marathon, she didn’t expect to win. In fact, when she took off, she thought she was running among the lead group of marathoners, until the course split. Then she realized she had jumped out with people competing in the half marathon.

“When I was out there at the start, I was kind of clueless,” she said. “I started running with the half.”

When she hit mile 21, she passed the turn that takes her toward her house.

“Oh, I’m so close,” she said.

Heading into the race she wanted to break three hours. She wanted to be among the leaders. She went ahead and won the whole thing.