HOUSTON - When little Jack Guyre was born, doctors at Texas Children's Hospital noticed something wasn't quite right.

"Without surgery, his chances of surviving to adulthood were zero," said Dr. Daniel Penny, chief of Cardiology at Texas Children's Hospital.

There was a hole in Jack's heart. It's part of a complex defect that crippled his blood flow. Multiple surgeries during his childhood kept him alive. Now, Jack is defying the odds.

"I feel like a normal kid," said Jack. But this 12-year-old boy is extraordinary and he has the hardware to prove it.

"When I crossed the finish line I nearly cried," said Jack. "I realized I did this in under three hours. It was really hard to compose myself in that moment."

Jack ran Houston's Half Marathon this year with his parents. He finished in 2:44:25, one second before his dad.

"It was so exhilarating to see him cross the finish line," said Emily, Jack's mother. "To see him do something people didn't think, or maybe he didn't think he could accomplish."

His cardiologist says he's an inspiration.

"Just because you have heart disease doesn't mean you have to play chess for the rest of your life," said Dr. Penny. "Jack has overcome great challenges that we thankfully haven't had to face. And he's come through those with great spirit and great determination."

With his first half marathon now under his belt, Jack says he's not done. "I'm already signed up for next year," said Jack.

He wants kids dealing with his same condition and families wondering if their kids can do the impossible to believe.

"It's more mental than physical, so if you have a good mindset you can basically run anything," said Jack.

Mind over body, pumping a zest for life into a 12-year-old's imperfect heart.

"I just hope they're inspired by him," said Emily. "They may not be able to do a half marathon, but maybe there's something else they want to achieve."