December 16, 2015 11:01 AM

Chris Floyd; Men's Health

John Krasinski is now a full-blown action star – with a body to boot!

Best known for playing The Office‘s lovable Jim Halpert, Krasinski switched gears to play a Navy SEAL in his new movie 13 Hours: The Secret Soldiers of Benghazi, which required intense training. He had to get ripped (aka shirtless-ready) in just 16 weeks, Krasinski, 36, revealed in the January/February issue of Men’s Health (on stands December 22).

But before he started training, he allowed himself one last blowout indulgence. To celebrate New Years Day 2015, Krasinski vacationed in Mexico with his wife Emily Blunt and some friends and indulged in 50 (yes, 50) quesadillas. “There was a create-your-own quesadilla thing, and this guy just kept making us quesadillas,” he explained. (Krasinski’s 4-month training schedule would start January 2.)  “I thought, ‘This is it. This is my last night of freedom.’ So I went for it. I thought, ‘Oh, it’ll all turn into muscle.’ That’s where I didn’t really understand the science.”

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Luckily, he had trainer Jason Walsh to whip him into shape, and they worked on burning fat while adding muscle. (When Krasinski got back from Mexico his body fat was at 26 percent, and Walsh estimated the actor got down to 5 percent by the time he filmed his shirtless scenes.)

“[T]o look like the guys you’d want to have saving you if you were in trouble,” as Krasiniski put it, he worked out twice a day, five times a week. “I gotta be honest: It was brutal at times,” said the star. “We did tons of metabolic work, dragging sleds and all this stuff I’ve seen NFL players do.”

Chris Floyd; Men's Health

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But the hard work paid off.  “Getting stronger every day is thrilling,” he said. “To be able to do 20 pull-ups.”

Chris Floyd; Men's Health

And Krasinski plans on keeping his buff bod.  “I think there’s a part of you where you become addicted to it. I love feeling strong,” he explained.”You pick up your daughter [he and Blunt have 22-month-old Hazel] with ease, while everyone else makes a little grunt when they pick up their kids. I walk with better posture and a little more presence. I was more of a sit-in-the-corner guy. Now I’m a lot less of that.”

–Michelle Ward Trainor

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