By Megan McCluskey
August 3, 2016

Matt Damon is back for the fourth time as Jason Bourne in Paul Greengrass’s film of the same name, but with 14 years under his belt since the series’ first installment was released, getting in shape to play the über fit super spy was apparently no easy task.

During a recent interview with Thrillist, Damon’s trainer Jason Walsh explained that the 45-year-old actor began training for the role long before filming began. “We didn’t just start prepping for Jason Bourne,” he told Thrillist. “We had two other movies before that [The Martian and The Great Wall], which started back in 2014.”

He went on to reveal one particularly-grueling activity that was incorporated into Damon’s fitness regimen while he was on the set of The Great Wall in the Gobi Desert. “We started doing sprints and long-distance runs a couple times a week, but we’d also do mostly metabolic sprints,” Walsh said. “They were pretty horrible, I’m not gonna lie. And we were at very, very high altitude. We both kind of looked at each other halfway through like, ‘What the hell is wrong with us?'”

And of course, the workouts didn’t get any easier when Damon was actually on the Jason Bourne set. “We would wake up in the morning sometimes and go for long jogs out in the park. In Tenerife [in the Canary Islands] we’d do them in the mountains,” Walsh said. “And that really sucked. Just running at a high grade — a 10 percent grade — is a grueling feeling. Your heart’s in your throat. It’s horrible. When we were in Tenerife, we would do five, six miles. We did some of that in London, too.”

Read more at Thrillist.com.

Write to Megan McCluskey at megan.mccluskey@time.com.

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