Jake Gyllenhaal: I'm Done "Exercising Regularly"

Fitness fanatic Jake Gyllenhaal is taking a breather.

In the September issue of Details, the 31-year-old actor says his notoriously grueling workouts are a thing of the past. "I haven't cycled in a long time," he reveals. "I don't run anymore. Do I take care of my body and take conditioning seriously? Yes. But exercising regularly doesn't fit the energy of the character I'm playing now," he says of An Enemy, in which he plays a man who seeks out his look-alike.

Gyllenhaal -- whose famous exes include Reese Witherspoon, 36, Kirsten Dunst, 30, and Taylor Swift, 22 -- says he's become a different man since he shot the police thriller End of Watch, directed by David Ayer. "Dave told me right off that this was going to affect my soul," Gyllenhaal explains. "'Friends are going to say, That's the Jake I've alw ays known, but somewhere deep inside, you'll know otherwise.'"

The actor spent five months prepping for the 22-day shoot, which included "three nights a week in ride-alongs with cops," Gyllenhaal recalls. "[We had] fight training every morning at a Kenbo Karate dojo, and I got the sh-t kicked out of me. Then [we went to] the shooting range, shooting past each other's heads, with live ammo."

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"There's a simulated fire in the movie, but Dave wanted us to feel what that's like, so he had us do a controlled burn," Gyllenhaal adds. One Saturday, the actor and his costar, Michael Pena, 36, "drove down to Orange County, dressed like firemen head-to-toe, and suddenly we're there jus t sitting in the middle of a burning building."

According to Gyllenhaal, the film -- in theaters September 28 -- inspired him to go with his gut.

"Every journey starts with fear. And I could say that's what I want to embrace now. A real experience," he tells Details. "And I want, overall, to trust what I know is right. There have been many times when I haven't. It's what I'm asking myself: Where is the line? What is the line? There's so much context, it can be almost impossible to find."

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