Fresh from bettering her 800m freestyle personal best at Sheffield’s Ponds Forge, Carlin was back in the water.
And the 22-year-old showed no signs of fatigue as she touched home first in a national record 15:47.27 minutes.
The time was not only well inside the qualifying standard for the Barcelona this summer, but it was also the fastest time seen this year by almost 14 seconds.
And Carlin couldn’t believe her luck after making the world sit up and take note once again, although she won’t start dreaming of repeating the feat in Spain just yet.
“I am really pleased with the 1500m and it went better than ever expected,” she said.
“Last year was really tough with the Olympic disappointment but when you come back and do best times like this it is all worth it.
“I’m obviously loving being back, it was tough after the 800m but I’m in good shape so it’s nice to know I could do back-to-back hard swims and to come away with a best time and British record, I’m over the moon.
“I always keep my feet on the ground, it’s about getting back into training, working hard again and getting ready for Barcelona.
“I want to be in the best shape that I can as I know I’ve got to face the best in the world – Olympic medallists and Olympic champions. I’ve got to be ready for that."
Also making waves is Ben Proud in the 50m butterfly, after setting the third fastest time in the world this season.
Proud had already broken the British record in the semi-finals and he bettered that again on his way to collecting gold – but he warned his World Championship rivals there is more to come.
“I broke the record in the semi, so I had a lot of pressure off in the final,” said Proud, who clocked 23.10 secs.
“I fixed my start from the semi in the final but I had a mistake towards the end because I glided to the finish, I didn’t time it right and that’s something to work on. It’s a huge shock to break the record.”
Elsewhere, Marco Loughran pipped Liam Tancock to gold in the 50m backstroke final, but both made the qualifying time for Barcelona.
Olympic silver medallist Michael Jamieson won the men’s 200m breaststroke final, Stacey Tadd claimed gold in the women’s 50m breaststroke, while Robbie Renwick and Eleanor Faulkner walked away with the men’s and women’s 200m freestyle titles respectively.