"I did not realize what kind of athletes that jockeys are," Maguire told reporters in California recently.
"They're just warriors, these guys. It's unbelievable," he continued. "The first time that I actually got up in the stirrups and did a bit of galloping on a race horse, and also just did some posting, I couldn't believe that after a couple of minutes my legs were noodles. I could barely stand up."
To prepare for the role, the "Spider-Man" actor also dieted down to a lower weight to give him more credibility as a jockey and even trained with retired Hall-of-Fame rider Chris McCarron.
"We brought in a mechanical horse that they call an Equi-ciser to my house and several times a week I would get up there, and Chris would coach me, and that was both for form and just to build up my strength and endurance so that I could do it."
Now available on video and DVD, "Seabiscuit" earned Golden Globe nominations last week for best drama and for best actor in a supporting role for William H. Macy.