The organizers of the New York City Half Marathon insist that they love the Manhattan Bridge. But they are forsaking it.

The 13.1-mile race on Sunday morning will take the Brooklyn Bridge. The runners will stream onto the Manhattan-bound lower roadway, not the wooden-planked pedestrian walkway on the upper level. It will be the first time that a race has gone over that bridge, according to the city Department of Transportation.

The new route is “anywhere from 30 seconds to a minute faster,” said Ted Metellus, the race director for New York Road Runners, which organizes the Half. The runners will not have to cope with a couple of uphills on the way to the bridge — and the Brooklyn Bridge is flatter than the Manhattan Bridge, he said.

So everyone — all 27,000-plus entrants — will set a personal record, because no one has run that course before.

Why is New York Road Runners making the change?

Construction in Lower Manhattan, according to Metellus — specifically, a part of the $1.45 billion Lower Manhattan Coastal Resiliency Project, which is intended to protect against flooding as water levels rise with climate change.

The massive project has made the section of the New York City Half’s usual route, from the Manhattan Bridge to the Franklin D. Roosevelt Drive, impassable for runners. Drivers would probably say the same. Only one lane on South Street is open during the day. And the bike lane has been closed completely.

The runners will start at Prospect Park, the same as always. They will run along Flatbush Avenue, the same as always — until just before Mile 4, when they will make a left turn onto Tillary Street and, a block or so later, a right onto what the city calls Brooklyn Bridge Boulevard, the elbow-shaped thoroughfare leading to the span.

There are other ways to get there, but “Tillary made the most sense,” Metellus said. “It’s a wide intersection. It’s big enough to clear the athletes safely, get them onto the bridge Manhattan-bound.” The change will put the runners onto the F.D.R. about a mile south of where the Manhattan Bridge would have.

Of the 27,000-plus entrants, 23,000 or so might notice the difference. New York Road Runners says that more than 4,200 will be running their first half marathon. The four defending champions have signed up: Abel Kipchumba of Kenya and Karoline Bjerkeli Grøvdal of Norway in the men’s and women’s open, and Geert Schipper of the Netherlands and Susannah Scaroni of the United States in the men’s and women’s wheelchair divisions. Also running will be Conner Mantz, who broke the American men’s record in the half marathon with 59:17 at the Houston Half Marathon in January. Kipchumba’s time in the New York City Half last year was 1 minute 10 seconds faster.

When it became clear that the route had to be changed, New York Road Runners worked up “seven or eight variations of courses,” Metellus said. “The Brooklyn Bridge was going to be our last option because we wanted to keep the sanctity of the course, which was having us go over the Manhattan Bridge.”

Will the runners move back to the Manhattan Bridge in 2026? Metellus called that “the million-dollar question.” He said the construction was scheduled to continue for at least 10 months. “So we’ll see where that plays into what decisions are made about where to go next year,” he said.

One of the awards in the New York City Half is the Commissioner’s Cup, based on the times of the top three runners from each of the city departments and offices that enter. Last year — as Ydanis Rodriguez, the transportation commissioner, noted — his department’s top three finishers were second, behind the top three from the Police Department and ahead of the top three from the Fire Department.

The commissioners of city agencies serve as honorary captains, and while Rodriguez is not running, the entrants from his department include Joshua Benson, a deputy commissioner whose portfolio includes the city’s speed cameras. They measure the speed of cars, not runners.