In the 40 years since Frank Shorter won the Olympic marathon in Munich in 1972 — yes, it has been that long — running in America morphed from the obsession of a few to a national fad to a mainstream form of exercise.

Drive pass any Minneapolis lake between 5 p.m. and sundown on a sunny spring afternoon, and you’ll find dozens of runners of varying ages and speeds, men and women, tooling around in singlets or spandex.

Today’s marathon fields match the populations of small towns, even cities. More than 22,000 people entered this year’s Boston Marathon, and despite searing heat all but about 800 finished. The New York Marathon draws more than 100,000 applicants despite a tough qualifying standard and an entry fee of more than $200; organizers cut off last year’s field at 47,000.

Even Grandma’s Marathon in Duluth attracts 10,000 runners, unthinkable growth from the 150 who ran the inaugural event in 1977.

That boom has long been credited to Shorter, now a 64-year-old grandfather who still trains about an hour every day.

On Wednesday, about 200 runners gathered at the grand opening of a Running Room store in the Eagan Promenade Shopping Center to run a 5-kilometer race for charity and meet Shorter, a longtime pal of store founder and author John Stanton. More than half the runners were women, and most appeared too young to remember Munich, when Shorter became the first American since 1908 to win Olympic marathon gold.

The son of a family doctor from Middletown, N.Y., about 70 miles north of New York City, Shorter went on to win the Sullivan Award as the nation’s finest amateur athlete. Mark Spitz, who pocketed seven swimming golds at the same Games, was ineligible because he won it the year before. Four years later in Montreal, Shorter overcame a stress fracture in his left ankle to finish second, the first U.S. marathoner to earn two Olympic medals.

“I was just an American who was raised in the standard fashion — the upward mobility, middle class family, trained to be a professional in terms of academics,” said Shorter, a Yale graduate who earned a law degree from the University of Florida.

“I started running to and from school when I was 10, 11 years old. I didn’t realize it at the time, but it was stress relief as much as training. Now millions of people do it.

“The winning and the medals I won at the Olympics were a byproduct of the routine I had gotten into. It’s not like I got into it with expectation of winning. I got into it because it was part of the lifestyle I developed. I just happened to be good at it.”

The stress came from something Shorter only began discussing in detail a few years ago — an abusive father who savagely beat Shorter and his siblings, and may have sexually molested two of his sisters. The Shorter family described it last year in this long Runners World piece.

“It was absolutely tied up in it,” Shorter said. “When I was young, people would see me running and go, ha-ha, are you running from something? And the answer was ‘Yeah.’

“One of the abilities you develop in those situations is the ability to tune things out. If I don’t want my mind to be there, it’s not going to be there. I never thought about the Israeli massacre or the terrorists during the (1972) race, and I never thought about my broken ankle in the second race. I just didn’t. It’s not that you’re in denial. It’s part of your survival mechanism.”

Curiously, the running boom produced recreational runners in big numbers but few with Shorter’s world-class chops. After his silver in 1976, the U.S. went 28 years without a men’s marathon medal until Meb Keflezighi, a refugee from Eritrea who became a naturalized citizen in 1998, finished second in Athens in 2004.

Now 36, Keflezighi ran a personal best 2:09:08 to win the U.S. Olympic Marathon Trials four months ago — a time that ranks 92nd in the world this year. Runners from Kenya and Ethiopia own the top 31 times. Among women, no U.S. runner appears in the top 20. Go to any big marathon in this country, and chances are the top finishers will be Kenyans.

“There’s a cultural difference,” he said. “In Kenya, running is a way out of poverty. That’s not the way it is for Americans.

“We don’t have the numbers, but we’ve never had the numbers. The American talent pool has never been that deep.”

Shorter also thinks the politics of running — pressure from agents, and conflicts between shoe companies and their clients — distracted top runners, who pursued making money more than running the fastest times.

“If you look at the good American runners in the past, there was a certain independence and individual aspect,” he said. “Steve Prefontaine, Kenny Moore, Alberto Salazar, people who had these independent streaks and goals that had to do with the achievement rather than what came after the achievement.

“People sort of lost track of the prize. What was the real objective here? My view of it was, if you want to make money, be successful — and then see what happens. But you can’t along the way let it influence what you’re doing.”

Shorter, who coached himself after leaving Yale, praised 2008 Trials winner and 2012 runner-up Ryan Hall for breaking that cycle by becoming his own coach. And Shorter hopes that more top Americans will train together, re-establishing the camaraderie he shared with Moore and Prefontaine that helped all of them excel in the early 1970s.

“I hope I sound hopeful,” he said. “They’re there. I think it’s just regaining focus.”