July 19, 2016 11:30 AM

Matt Damon is the first to admit that it wasn’t easy getting back into shape for the new Jason Bourne movie.

The actor, 45, revealed that he was all but told he needed to really bulk up to reprise the role as the former CIA assassin – and judging from multiple shirtless scenes in the upcoming film, that’s exactly what he did.

“It was a lot of work to get back to that, but we felt it was really important,” Damon told PEOPLE Monday at the premiere of the film in Las Vegas. “[Director] Paul Greengrass said to me really early on, ‘If this movie starts and you look like you’ve lived well for the past decade, we have no movie,’ so that was really the motivation.”

Damon said the hardest part of getting back into shape was having to maintain a steady workout plan while filming.

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“I still had to keep going to the gym with horrifying regularity to stay in shape, so that was the challenge, along with the long work hours,” he said.

He added, “I was in shape for way longer than someone my age should be.”

The fifth film of the franchise follows Bourne as he finally remembers who he truly is, and tries to uncover the truth about his past.

Jason Bourne hits theaters July 29.

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