Over 23,000 competitors left the starting gate at the Rock ‘n’ Roll Half-Marathon in Philadelphia this morning.

At the end, however, there was little question that the day belonged to one woman.

Deena Kastor set the World Record in the Women's Masters division for the half-marathon with a finishing time of 1:09:39. Kastor, who currently resides in California, holds numerous half-marathon records—including a 1:07:53 finish in Philadelphia in 2005. But it was clear this one meant something special.

"I set a couple records at a race in Dallas earlier this year," Kastor said, "and I think that performance kind of motivated me to elevate my game, to put in the extra work. I've been training all summer with my Mammoth Track Club teammates."

Setting a world record would seem to require the best day of your life. You need everything to break just right—perfect training, perfect weather and the performance of your life. But Kastor's Sunday morning performance was quite the opposite. While the weather was pretty ideal, Kastor hit a snag around the halfway point.

"I didn't think I'd cut it this close," she admitted. "I got a side stitch at about six miles, so I slowed down to try to breathe it out. Sure enough, it came back in the last couple of miles. It's something you don't anticipate… but it took that little discomfort to make me hunker down just a bit."

Kastor pulled no punches about her focus on breaking this record, saying she was tracking her time for about the last five kilometers. "I went back and forth—I've got it, I don't have it," she recalled. "I went through that mental battle—should I give up? Should I dig deeper? But you don't get that many chances, so I went for it."

She made it with 17 seconds to spare, beating the old record of Russia's Irina Permitina. Minutes later, in the shadow of the steps where Rocky Balboa completed his famous training run, Kastor discussed setting the record here in Philadelphia.

"This is an iconic running spot, and a great race," she said. "This Rock 'n' Roll series has done so much for both professional athletes and for people looking to get up off the couch and be active."

Kastor said she doesn't really track career highlights—"every year gets better"—but allowed that it would be tough to top this one. Of course, she's going to attempt to do so anyway.

"I'm doing the New York Marathon in November," she said in closing. "I'm going to get home and start preparing for that one."

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