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.

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

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 soon will 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, the state holiday of Patriot’s Day, the 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.

“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.”