J. Lo's Triathlon After Twins!

updated 09/08/2008 AT 01:00 AM EDT

originally published 09/08/2008 AT 01:00 AM EDT

A former track star, "Jennifer is naturally predisposed as a runner," says her trainer Gunnar Peterson. But to improve her stamina after having twins Max and Emme in February, Lopez, 39, alternated quarter-mile walks and sprints, slowly increasing her running distance to the four miles required for the Sept. 14 race.

To get ready for the 18-mile ride, Lopez—who is raising money for the Children's Hospital Los Angeles—has been cycling "30 to 50 minutes at different intensities" on a custom bicycle, says Peterson, who trains Lopez five to six times a week for 45 minutes to two hours each session.

To prep for a "killer" half-mile swim in the ocean, Lopez first swam 650 yards in a pool before upping the ante "distance-wise and intensity-wise," says Peterson. "She will be ready on race day." Lopez told Good Morning America: "If I have to crawl across that finish line, I'm going to crawl across that finish line!"

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