In a country often lampooned as populated with obese soda-swilling TV junkies, some 9.2 million people completed a certified foot race in the United States in 2008, up from 3.7 million in 1987.
Of those, 425,000 completed a marathon -- 26.2 miles, or 42.2 kilometers -- and 715,000 ran a half-marathon, according to Running USA, a non-profit group that promotes running. That's up from 143,000 marathon runners in 1980.
The numbers are expected to be even higher this year, said Ryan Lamppa with Running USA. "There is still a pent-up demand for races in the country," Lamppa told AFP.
Marathons across the country are filling up so quickly that race organizers are adding half-marathons (13.1 miles, or 21 kilometers) along with shorter races on event day, Lamppa said.
Some 40,000 people ran the New York marathon in early November. In late October some 32,000 people ran the Marine Corps marathon in Washington DC, and some 45,000 ran earlier in Chicago.
In Atlanta, 55,000 people signed up for the November 26 marathon -- the bulk of the tickets sold online in seven hours -- and some 45,000 are expected at the Walt Disney World Marathon Weekend on January 10 in Florida.
Why the growth? Running is the cheapest, fastest way to lose weight, and along with walking, the easiest way to exercise.
But that's only part of the answer.
We live in a financially uncertain, violence-scarred world, and running "gives you something to control -- you can't control the stock market or the economy, but you can control your health," said Lamppa.
Michael Giordana, a sports sociologist at the University of Illinois, Urbana-Champaign, believes there are three aspects fueling the running boom: people inspired by the 2008 Olympics, increased social networking -- for example runners opening Facebook pages to collect money for charity -- and a reaction to what he called "the obesity epidemic."
According to the US Centers for Disease Control, about one-third of American adults are obese, while another third is overweight.
As more people run and enjoy the experience, good word-of-mouth attracts new runners. "The stories that come out motivate people to get off the couch and be more active," said Giordana.
Race days have also become city-wide carnivals, complete with live music, free food, street vendors, and crowds cheering on the athletes.
Big sponsors have jumped into the act. While schools and local stores focus on neighborhood 5K runs, marquee names like Bank of America, ING, McDonalds and Continental Airlines have sponsored major races this year.
Training is also widely available, Giordana said. Aside from scores of books on running, there are software programs and training programs held at health clubs, some catering to specific interests like religion or single runners looking for a partner.
Runner's World magazine, the sport's bible, held a virtual training program through its website that culminated in the November 14 Richmond, Virginia marathon.
Need in-person support? Bridget Bowers heads a training program run by Pacers, a northern Virginia running shoe chain store.
"It's a very social thing to do," said Bowers, who also coaches the American University cross-country team.
Training programs usually attract more women, especially ages 30 and above, than men, said Bowers. Women also tend to be more committed to the training, she said.
Women have been fueling the growth in the number of runners: while in 1987 21 percent of those running were women, by 2008 it was 50 percent. Looking at marathons alone, the numbers have jumped from 10 to 41 percent over the same period, according to Running USA.
With all the knowledge available "it has become more realistic -- women can do more marathons, and are good at it," said Bowers.
Running is also an especially democratic sport, as almost anyone, of any age, can participate with a small investment in running shoes and clothing. As more people run and enjoy the experience the sport "has reached mainstream America," said Lamppa.
But unlike the wiry racers of the 1980s, most of today's runners are interested in completing a marathon, not competing in it, Lamppa said.
Marathon winners usually clock in around the 2:20 mark, some runners aiming to just finish the race plod across the finishing line after six hours or more. Many races have buses that pick up the extra-slow runners, as keeping a road closed can be expensive.
Bowers has mixed feelings about the trend. "At that point you're not really running a marathon, you're walking it," she said.
But if people "are moving, that's great -- our country needs exercise," she said.