Fifteen minutes after the Broad Street Run online registration opened Wednesday morning, the website was gasping for air.

"It's been nuts," James Marino, director of the race, said at lunchtime. By noon, 13,000 runners had persevered, repeatedly refreshing their computer screens until they got through and signed up.

By 3 p.m., it was all over.

An apology appeared on the home page: "Registration for the Blue Cross Broad Street Run has exceeded our wildest expectations with over 30,000 runners registering in a record five hours."

The run, sponsored by Independence Blue Cross and the Philadelphia Daily News, among others, was started in 1980 by the Department of Parks and Recreation, which continues to run the event. That year, 1,500 runners completed the race, which raises money for the American Cancer Society.

Last year, registration reached capacity in four days, and 25,000 made it across the finish line.

The event's popularity has taken off in part because the city's population of young and fit college students and graduates has increased.

Recent census data show that 393,000 young adults, ages 20 to 34, now make up a full 26 percent of the overall city population. That's an increase of more than 50,000 people in the last decade.

"That's definitely our demographic," said Marino.

Last February, an article in Forbes about the top magnet cities for the college-educated crowned the Philadelphia region as No. 1 among Northeast urban centers of more than five million people.

"I've always thought Philadelphia's ace in the hole is its authenticity, its lack of pretension," said David Thornburgh, director of the Fels Institute of Government at the University of Pennsylvania.

In the mid-2000s, Thornburgh served as director of the Pennsylvania Economy League, which produced a study showing that the Philadelphia region was losing this critical element of the population and failing to retain the large numbers of students enrolled in colleges and universities here.

The Broad Street Run is just one measure of an apparent reversal in that trend, particularly in Center City.

Tuesday evening at 6, nearly 200 people, most in their mid-20s and early 30s, jammed into Philadelphia Runner, a small shop at 16th and Sansom Streets that sells running gear.

It was the store's second annual singles Valentine's Day run, a three-mile post-work loop for unattached men and women, ending in free drinks at a local bar, Ladder 15.

"It's a fun way to meet people," said Amanda Dougherty, 27, a tax lawyer for the city, after slipping into a pink high-tech T-shirt with a heart logo on it, part of the $25 package deal for signing up for the run. She and friend James Vandermark, 32, also a tax lawyer for the city, entered the event, hoping to find love, but if not, at least to get in a decent workout and knock back a free beer.

Dougherty, who grew up in Oklahoma, and Vandermark, who comes from a small town near Erie, met in law school at Temple University and decided to stay in the city after graduation.

"If you go out running on Kelly Drive in the morning, it's packed with young people," said Vandermark.

Philadelphia Runner has been in business for eight years. Around the corner on Walnut Street, there's been a proliferation of retail stores specializing in athletic gear, joining the well-established City Sports and Puma.

Like Philadelphia Runner, many of the stores provide classes for customers and help local fitness companies advertise. Flyers for yoga and Pilates classes overlap on the bulletin board at Lululemon Athletica, which offers its own yoga classes and organizes runs. Athleta, a Gap subsidiary that opened a store in the 1700 block of Walnut in November, has a calendar of free classes in ballet, yoga, and meditation, with organized runs as well.

Earlier in the week, a few shops offered reminders about registration for Broad Street. Unnecessarily, it turned out.

The run, which will be May 6, has become a massive cattle chute, with throngs of runners of all shapes, ages, and abilities descending on Broad Street near Albert Einstein Medical Center. They line up by the hundreds at a brace of porta-potties, jostle into the designated corrals - based on estimated minutes-per-mile pacing - and hydrate on bottles of electric-orange and neon-blue Gatorade.

The course, a straight shot, gently downhill, ends at the Navy Yard, where the runners are greeted by cheering crowds, bananas, potato chips, and bottled water.

The event's organizers said Wednesday they would try to accommodate more runners through a second-chance lottery.

The link, posted on the front page of www.broadstreetrun.com, will be open through Feb. 24. The following weekend, 2,500 will be chosen.

Don't hold your breath.

Contact staff writer Melissa Dribben at 215-854-2590 or mdribben@phillynews.com.