Since she last lowered her world record in the women’s 400-meter freestyle 24 months ago, Katie Ledecky has graduated from high school, enrolled at Stanford, had her wisdom teeth pulled and blossomed into the dominant female swimmer of her era. She has lowered her own world records in the 800 free by nearly 4 1/2  seconds in that span and in the 1,500 free by almost nine — but the 400 mark had withstood every challenge from every swimmer capable of taking it down, of which, in truth, there was only one.

On Sunday night, in her first individual final of the 2016 Rio de Janeiro Olympics, 19-year-old Katie Ledecky finally took down 17-year-old Katie Ledecky, her only real competition, in the 400. And it was an absolute rout.

Ledecky, the freestyle phenom from Bethesda, won her first gold medal of these Games, torching the field in the final of the 400 free at Olympic Aquatics Stadium with a stunning time of 3:56.46 to win by nearly five seconds over Britain’s Jazz Carlin. U.S. teammate Leah Smith took the bronze. Ledecky punctuated her swim with an unbridled fist pump, followed by a giant smile.

“We set our goals for the week,” Ledecky said, “and to finally hit one of them feels really good. Everyone wants to swim their best at the Olympics. I was just trying to set a [personal] best time, and for me that just happens to be a world record.”

Katie Ledecky developed the "gallop" in her stroke with former coach Yuri Suguiyama. It gives her an advantage in the pool over her female competitors since most do not have the strength to sustain it. (Jayne Orenstein/The Washington Post)

With the 12th long-course world record swim of her career, Ledecky finally took down her 2014 mark of 3:58.37. It was apparent the old record’s hours were growing short after Ledecky clocked a 3:58.71 in the afternoon preliminary heats. Though she paced the qualifying by more than four seconds, a “lackadaisical” — her word — final stroke may have cost her the record.

“That’s the easiest it’s felt going under four minutes,” Ledecky said after that swim. “So that bodes well for tonight.”

When Ledecky says something “bodes well for tonight,” the swimming world knows to be on world record alert. In her first three swims here — a pair of blistering 100s as the anchor leg in the prelims and final of the 4x100 free relay Saturday and her flirtation with her 400 free world record Sunday afternoon — she hinted at the potential for a historic week at these Olympics. And Sunday night’s massive swim drove it home.

“It felt like a pretty similar swim to this [afternoon], and I knew I had a lot left at the end this [afternoon],” she said. “And I just let it all out.”

Ledecky’s intentions were clear from the beginning Sunday night. Her opening lap of 27.73 was more than a half-second faster than her afternoon split, and her first 100 was more than a full second faster. Midway through the race, she was already two body-lengths ahead of the field. She had churned through her first 200 in 1:57.11, a time that would have given her fifth place in the 200 free at the U.S. Olympic trials — and at least in theory, she was still saving something for the back half.

“I’ve been training with her for the last month and watching her doing amazing things,” Smith said of Ledecky. “I knew this was coming. It was just a matter of when and how fast she was going to go.”

Katie Ledecky's journey to becoming an Olympic gold medalist

But it was at the end of the race that Ledecky switched to a gear no other swimmer in the field — and perhaps no middle- or long-distance swimmer in history — could ever hope to possess. Kicking furiously and churning her arms with brutal efficiency, she traveled her final 100 meters in 58.84 seconds. No other swimmer in the field was under a minute. Neither, for that matter, was the 17-year-old Ledecky in the world-record swim from 2014.

Asked when she started thinking about the world record, she said, “Just a little bit throughout” — then added: “But I don’t like to think too much.”

Of Ledecky’s three world records, the one in the 400 had stood the longest by far. She broke it twice within a span of two weeks in August 2014 — the second coming at the Pan Pacific Championships in Australia — but she had not bettered it in the two years since despite taking another 20 or so cracks at it. In that same span, she has broken her records in the 800 and 1,500 (the latter of which is not contested for women in the Olympics) a total of five times — most recently six months and one year ago, respectively.

Her gold medal performance Sunday night, added to her 800 free gold in London in 2012, already makes Ledecky just the sixth American woman to win individual swimming golds at two different Olympics — joining Martha Norelius (1924 and 1928), Janet Evans (1988, 1992), Brooke Bennett (1996, 2000), Natalie Coughlin (2004, 2008) and Rebecca Soni (2008, 2012).

Still to come are additional gold chances in the 200 free, 800 free and 4x200 free relay. Win them all, and she would be only the third American woman, after Amy Van Dyken (1996) and Missy Franklin (2012), to win four golds in a single Olympics. Only Franklin has won four golds plus an additional medal (a bronze).

Next up for Ledecky: the 200 free, with heats Monday afternoon, semifinals Monday night and final Tuesday night. Though she held the top spot in the world rankings for much of the year with a 1:54.43 from January, she was passed last month by Sweden’s Sarah Sjostrom (1:54.34). Their expected duel for the gold here in the 200 is one of the most anticipated races of the Olympics meet.

But already this week it has become clear: The Ledecky of August 2016 is not the Ledecky of 2014 or the Ledecky of January or even the Ledecky of June’s Olympic trials. Right now, she is better than all of them.