Griffin Mazeski, a 9-year-old from the Denver area, just set a single-age record for running a half marathon in 1:33:43.

He set the mark at America’s Finest City Half Marathon and 5k last weekend. That’s about a minute faster than the record set by a 9-year-old from St. Pete Beach, Florida according to the Association of Road Racing Statisticians.

“It was pretty hard and I didn’t know if I could really break the record because I didn’t feel that prepared coming into it,” Griffin told 9News in Denver.

The family of four started the race together, but Griffin and his dad, Brian, eventually broke away from his mother and 12 year-old sister. The father-son pair ran 11 miles together before Griffin pulled ahead and finished the last few miles solo.

“The first 11, I mean, I felt really good and I was going under my pace that I wanted and I felt really calm and I could breathe easily, but at mile 12 there was a really long hill and I sort of died out on that,” said Griffin, who turned 10 on Aug. 23.