1. He worked out six hours a day, every single day.
Gyllenhaal’s pre-movie routine started light: 90 minutes in the morning, and again at night. Yeah, it's light by comparison: He eventually doubled this and was exercising six hours a day. His frequency and amped-up hours of intense training are what allowed his body to make such a dramatic jump in a relatively small amount of time (just six months).
2. He did more than just box.
While he worked for a solid three hours each day on boxing technique and footwork, Gyllenhaal also chiseled his physique with 2,000 daily sits ups, plus would flip 350-lb. tires and run eight miles five times a week.
3. He made sacrifices.
While Gyllenhaal didn’t have a particularly rigorous diet to match his crazy workout routine, he did eat relatively healthy and, quite specifically, avoided chicken wings.
4. He studied up.
Gyllenhaal has long been a student of boxing. He has attended several professional matches which he used as a major source of education for both his character and his workout regimen.