BOSTON — It’s been decades since you could run in the Boston Marathon without qualifying, before limits on the field size made entering — almost as much as finishing — something to aspire to.

The course has changed a dozen times or more. Women were officially welcomed in 1972, wheelchairs three years later, and prize money was introduced in 1986, ushering in a professional era that rejuvenated the event and fortified its status as the world’s most prestigious road race.

But nothing in more than a century has done more to shape how the Boston Marathon is perceived and how it will look in the future than the twin explosions at the finish line in 2013.

And when the field of 30,000 leaves Hopkinton on Monday for the 119th race, the effect of those bombs will be seen not just in the ever-watchful security but in the way the runners and their supporters have responded to the unprecedented attack.

“I don’t think it’s ever going to be just a race again,” said Desiree Linden, who returns this year in search of the American victory she missed by 2 seconds in 2011. “There’s so much history here: some of it is good, some of it is bad. When you run Boston, that’s always going to be a part of it.”

Over the more than a century since the first Boston Marathon in 1897 until Lelisa Desisa won in 2013, the event transformed from a footrace among friends into one of the world’s premier athletic contests.

But not until the bombings that killed three people and wounded 260 did the marathon became a touchstone for the resiliency of a city and its signature sporting event.

Last year’s race became the centerpiece of the city’s recovery, and the calls to take back the finish line were answered when Meb Keflezighi became the first American man to win since 1983.

“The marathon gods blessed Meb with that run. It was electric,” said Shalane Flanagan, who finished seventh last year and hopes to break a 30-year drought in the women’s race. “Last year was extremely special, just being an American. It’s a run I’ll never, ever forget.”

A daughter of marathon runners, Flanagan grew up in suburban Marblehead with a reverence for the Boston race. Just to run it was life-changing, she said; to win it would be an honor.

“Yeah, it was a race. But at the same time it was beyond a race, because of what was on the line,” Keflezighi said this week as he prepared to defend his cathartic 2014 title.

“We can’t get those people back; it can never be forgotten. It can never be normal, because everyone’s going to think about that moment. But we do what we can,” said the two-time Olympic silver medalist who had written on last year’s race bib the names of those killed.

Reminders of the April 15 bombings are still easy to find two years later.

Earlier this month, a federal court jury convicted Dzhokhar Tsarnaev of all 30 counts in the bombings and the manhunt, in which an MIT police officer was killed. Jurors will soon decide whether he should be sentenced to life in prison or to death.

Mayor Marty Walsh declared a day of remembrance and community called “One Boston Day.” On race day, already the state holiday of Patriots’ Day, the Boston Red Sox will wear special uniforms with the city’s name on their chest.

Security along the route has been increased. More miles of fencing between the runners and the fans. More officers on bicycle. Runners will again pack their belongings in see-through bags. Spectators will be screened before entering the finish-line bleachers.

“In some ways the plan is even deeper this year than it was last year,” said Kurt Schwartz, the Massachusetts undersecretary for homeland security. “Last year we built something completely new. We didn’t get it 100 percent right, and we figured it out along the way.”

Schwartz said officials have avoided more drastic measures, like creating a buffer zone between fans and the runners, or closing off certain areas of the course to spectators entirely. “It would just so fundamentally change the character of the day. It would be short-sighted,” he said.

“Last year, we still put a million or more spectators along the course. They were right along the street’s edge,” Schwartz said. “I don’t think the experience of the spectators or runners was significantly different last year.”

Everything else has changed.

Amby Burfoot, who first ran in 1965 and won the 1968 race, has watched the event grow from fewer than 500 men to an international spectacle. He has seen East Africans dominate since the race turned professional, and the addition of women, who this year will fill a record 46 percent of the field.

And he has seen the race respond to an unprecedented attack.

“Last year was without question the greatest footrace in the history of humankind,” said Burfoot, who is now an editor at Runner’s World magazine. “Every runner and every spectator was a hero last year. We can’t do that again. There’s only one of those.

“This year is almost a return to the new normalcy.”

Boston Athletic Association President Joanne Flaminio said there was so much pressure last year on everyone — organizers, competitors, security — to produce an event that would help people overcome the calamity.

“I think this year it’s different. We’re looking forward to a new chapter in our history and the next 100 years,” Flaminio said.

Consistent through that history, before the bombs and after, are themes like patriotism and resilience. Of overcoming pain and injury. Of amateurs running for charity, or just to make it to the end.

“The bombing is part of the Boston Marathon history now,” four-time winner Bill Rodgers said. “But I think the public got to see what the Boston Marathon really stands for, and how the Boston area came together.

“The healing is occurring; that’s what everyone wants. They want it to be a wonderful celebration, just like it has always been. And I think that’s what’s happening.”