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It takes a certain kind of person to do a marathon.

It takes another special type to run a marathon in “progressively worse places,” like a 1.38-mile course simply going back and forth across the Williamsburg Bridge.

But that was the initial idea behind the Williamsburg Bridge Marathon, an unofficial event now in its fourth year with no permit, no entry fee — and no predetermined distance.

This past Friday, well after sunset, runners lined up on the Manhattan side of the bridge, but wouldn’t know how far to go until after they’d started.

A random computer-generated number between 10 and 100 would be posted on the event website, letting them know how many spans of the bridge to complete (and log on the fitness app Strava).

This year, it was 17 bridge spans (approximating 21.42 miles) a disappointingly short year, according to race organizer and creator Matt White.

“Hopefully the random number generator gives us something harder next time,” White told The Post.

That being said, there were still plenty of DNFs (“did not finish”), but 13 women and 55 men completed the race, with Maya Geller taking top prize for the ladies (with a time of 3:15:04) and Joe Cady coming out on top for the guys (2:30:13).

After failing to qualify for the New York City Marathon, White decided on a whim to create his own race.

“We had gone out for drinks the night before, and I was like, ‘Yeah, I bet I can do it,’” the 31-year-old software engineer told BK Reader last year. “I really liked the idea of continuing to do marathons in progressively worse locations.”

After failing to qualify for the New York City Marathon, White decided on a whim to create his own race.

The first year, around 20 runners came out. This year, hundreds gathered for the strange event, with race volunteer James Cashen estimating anywhere between 300 and 400 attended for the same reason many people start running — a sense of community.

“To me, the race is special because it represents the best parts of New York and the running community,” Cashen told The Post. “It’s gritty, difficult and continues to build around the energy of fun-loving NYC runners.

Many participants sign up because of the absurd nature of the race, with previous participants leaving reviews like, “Not finishing this race was a win in itself,” and “This is not actually a race — more of a group experience in mutual pain.”

It also speaks to the community-building group running events lends itself to.

“I like that you can go to the bridge at almost any time of day, and there will almost always be groups of people doing radically different things,” White told BK Reader.

Similarly, Nathan Yan ran the course for the first time last year, right after he’d moved to the city, in search of a “high-effort ridiculousness” endurance event where he’d find his people.

“It was one of my very first ‘New York’ experiences,” Yan told The Post. “I just moved from Colorado where [I] used to do things like 36-hour night skiing marathons — so this was right up my alley.”