CHICAGO — Thousands of runners descended on Grant Park on Sunday morning for the 43rd annual Shamrock Shuffle.

The event, an 8-kilometer run and 2-mile run/walk, kicked off at 8:30 a.m. at Columbus Drive and Monroe Street with the first wave of runners. A second wave was scheduled to go off at 9 a.m., followed by a wave of walkers at 9:30 and a post-race party that ends at 1 p.m.

For more than four decades, the Shamrock Shuffle has been the traditional kickoff event of Chicago’s running season.

More than 24,000 runners from various states around the country signed up for Sunday’s event. Eighty percent were from the City of Chicago and its suburbs.

“I’m not exactly sure, but it’s an amazing tradition,” said race director Carey Pinkowski when asked how the Shamrock Shuffle got started. “For 43 years, this has been the rite of spring, the kickoff to the running season. So everybody’s ready to go.

“… We’re just looking forward to a great day. Some great athletes up front. Going to be very competitive.”

In the 8K race, which is almost exactly five miles, the elite runners finish in about 23 minutes. Zach Panning, who won last year’s 8K in a blazing time of 22 minutes, 48 seconds, was back Sunday to defend his title.

This year’s Chicago Marathon is scheduled for Oct. 13, so the road to that began Sunday with the Shamrock Shuffle

“This is our natural progression through our Chicago Distance Series,” Pinkowski said. “… Some of the athletes that you’ll see competing (Sunday), we’ll see them in the fall in the Marathon.”

But the Shamrock Shuffle isn’t just for serious long-distance and marathon runners. First-timers can do the 2-mile run/walk, and the event also included a mile run on Saturday.

“We had a bunch of kids, first-time runners, walking, kind of getting used to it, getting moving,” Pinkowski said.

“I think so often in running, we think of physical fitness,” said Deena Kastor, the 2005 Chicago Marathon champion and the American female 8K record-holder. “But I think more so, we’re seeing the science behind the mental fitness and the mental health that can benefit through an active lifestyle.

“So it’s great to see the lives impacted by that.”

Additionally, many participants in the Shamrock Shuffle run for charity or to raise awareness for specific causes.

“There’s just so much to celebrate in the running community, and these races are life-changing events for a lot of people,” Kastor said.

All of Sunday’s participants, whether serious runners or recreationalists out for fun, were grateful for the cool, dry weather. It’s not always like that this time of year in Chicago.

“It was 2009, and six inches of slushy snow on the course,” Kastor recalled of one particularly bad weather day for the Shamrock Shuffle. “It was one of the colder races I have ever run, but probably one of the most memorable, as well.”

There was no slush to contend with this year.

“Runners beg for a day like this,” Kastor said.