Blake Lively trained like a beast for her latest film.

“Being in waves like that, swimming like that, doing such long takes like that all the time — I became so much stronger and more fit by the end of production. I was working out 13 hours a day because shooting was working out,” the actress told Entertainment Tonight.

Lively, 28, who portrays a bikini-clad surfer fighting for her life against a great white shark in “The Shallows,” also revealed she was surprised to see her body bounce back after giving birth to daughter James in December 2014.

“It was neat to be able to do that, and it was neat to have that challenge after having a baby, because you think your body is so different — you think no matter what, it’s never [going to be] totally the same again,” Lively said.

“And to be able to be in better shape than I had ever been in before. I thought, ‘OK, that’s really encouraging,'” she continued.

While Lively credited trainer Don Saladino for kicking her butt “in the best way, in the most healthy way,” husband Ryan Reynolds, 39, provided plenty of support, having starred in a survival thriller of his own with 2010’s “Buried.”

“He always talked about ‘Buried’ and what an important thing that was for him to be able to do. What a challenge it was and boy, was it a payoff,” Lively shared.

“[For this film], he said, ‘Be prepared, this movie is a physical challenge, it is an athletic event,'” she continued, “like training for a marathon.”

This article originally appeared in the New York Post's Page Six.