Irvine - Six years on from her stunning 800m freestyle victory at the 2012 Olympics, Katie Ledecky is savouring her status as the grand dame, at 21, of distance freestyle.

Ledecky was 15 when she led wire-to-wire in winning the 800m free in London in a race that saw world record holder and British home hope Rebecca Adlington relegated to third.

Four more Olympic golds in Rio two years ago, 14 world titles and 15 world records later, Ledecky heads into the Pan Pacific Championships starting on Thursday in Tokyo knowing she's now the swimmer with a target on her back.

"I guess it's a little different," said Ledecky, but she knows that young rivals such as Australian distance phenom Ariarne Titmus and Canadian Taylor Ruck still have a lot of catching up to do.

"I know that there are a lot of great swimmers out there that are chasing me," Ledecky said.

"That motivates me just as much as chasing someone motivated me when I was 15."

Ledecky has made a seamless transition this year from US amateur collegiate swimming to the professional ranks, inking a sponsorship deal with TYR that the manufacturer trumpeted as "the most lucrative partnership in the history of the swim industry."

Ledecky specifically chose to make the move two years before the Tokyo Games, to give herself time to adjust to the new demands on her time.

While some US swimmers struggle with newfound responsibilities, Ledecky has taken it in her stride.

"It hasn't been extremely different," Ledecky said. "I guess my practice schedule has changed just a little bit."

Jon Urbanchek, the veteran coach recently named the US national team's technical adviser, calls Ledecky "probably the perfect role model" with her fabled work ethic and determination.

"She's on a mission," Urbanchek said.

"You can't really distract her."

Ledecky lowered her own world record in the 1 500m freestyle in May and came away from the US championships in California in July with victories in the 200m, 400m and 800m freestyles.

She opted out of the 1 500m freestyle, which she can still swim at the Pan Pacs, preferring instead to get a day of rest ahead of another burst of training before heading to Japan.

"I feel like I'm in a really good spot," Ledecky said - an innocuous sounding remark that will strike an ominous note for her rivals.