While it may have appeared that Gyllenhaal, 34, transformed into a mountain of muscles overnight to play fictional boxer Billy Hope, the actor underwent intense training to gain the body and mindset of a fighter.
"My concern was to look like a boxer," the actor tells PEOPLE in this week's issue. "The fear of looking like I didn't know how to box [on screen]" drove Gyllenhaal to dedicate the next months of his life to training twice a day, fully immersing himself in the life of a professional boxer.
In order to accomplish that goal, Gyllenhaal teamed up with trainer Terry Claybon, who tells PEOPLE he had to start from the bottom with the actor, who had no previous boxing experience.
"We started from the ground level, which is the perfect boxing stance," Claybon says. "We started him off with great footwork, great defense, and then we went into sharp, direct punches."
Throughout the eight months of training, Claybon gradually increased Gyllenhaal's workout regimen, focusing on his boxing skills and strength training:
Each day started off with jumping rope for 15 minutes straight.
Then the actor did three rounds (for a total of nine minutes) of footwork defense drills.
Also in the mix was a forward-step drill focusing on learning different punches and combinations – six rounds for 18 minutes.
"We would go into heavy bag drills with him hitting the bag strap, which is designed to guide you to perfect punches. He would also go into double-end bags, which move back and forth, and speed bag drills – three rounds each," Claybon says.
Sit-Ups, Pull-Ups and Dips
Gyllenhaal started with 500 sit-ups a day, but by the end, he was up to an impressive 1,000. He also included 100 pull-ups and dips into this routine.
"We had a 300-lb. tractor tire that we would have to flip over 20 times. Then we had a sledgehammer drill where he's hitting the tire with a sledgehammer for three minutes nonstop – that was a really intense drill," Claybon says. "We also had squat drills where he was doing speed squats for two sets of 100."
Claybon required Gyllenhaal to run five miles every other day, but because the actor was a natural runner, "five days a week he was running eight miles," Claybon says.
RELATED VIDEO: Jake Gyllenhaal Reveals His Secret to Getting Ripped for Southpaw
"He gained 15 lbs. of muscle," Claybon reveals. "If you train like a boxer, your body is going to be sculpture like boxers'."
For much more on Jake Gyllenhaal, pick up the latest issue of PEOPLE, on newsstands Friday