Michae l Peña wasn’t prepared for the rigorous training that went into becoming a police officer on L.A.’s mean streets in End of Watch, out Friday.

Rehearsals started about two years ago when his son Roman was 2.

“I was kind of chubby — just sitting around playing Legos or whatever,’’ says Peña from The Mandarin Oriental Miami Hotel. “Then they made me work out 10 times a week, once in the morning and once at night. Oh, God. I was exhausted for five months.”

To give the film a more authentic feel, preparation was essential with workouts including martial arts sparring and boxing; ride-alongs with real cops; weapons training; and lugging around chest cameras.

“Those were so were heavy, they were gnarly,’’ Peña says of the body cams. “But I think they paid off. The film is pretty intense and innovative.”

All that work and the shoot only lasted 22 days. Some of the hardest parts of the gig weren’t exactly physical.

“I was driving [the patrol car] a whole bunch. Then you have the director [ David Ayer, Training Day] in back, which can be pretty nerve-wracking. Sometimes I didn’t know where life began and where the acting started.”

Though Peña and his partner, played by Jake Gyllenhaal, have realistic conversations that cops would likely have during a long shift or “watch” — they insult each other endlessly but exchange serious advice when it comes to career, love and marriage. Barely anything was improvised.

“All that hard-hitting dialogue, it felt like a real brotherhood so you could be invested in those characters,” says the Chicago native, 36. “That was what we wanted to make it not boring. Also you don’t want to mess with [Ayer’s] lines. Nine times out of 10 you aren’t going to come up with something better.”

Being directed by Ayer, who also wrote the screenplay for 2001’s original The Fast and the Furious, was a trip.

“He was on a rampage,’’ says Peña, laughing. “He started putting little cameras on sticks to get like all these angles. The craziest thing I had seen before was a camera on a crane. He was really kind of adamant about it being new and fresh.”

Ayer knew the capabilities of his leading men.

“He pretty much said, ‘Mike, I’m going to have you give me the best performance of your life. Jake, you too.’ He studied our careers. We couldn’t do any tricks. He filled us with all these ideas about our characters.”

Peña saw the finished product at the L.A. premiere and was “floored.”

“It came out better than I thought it would, and you can’t say that too many times about your movies. I was like, ‘Wow.’ ”