(people.com) - It's only been six months since she gave birth to twins, but Jennifer Lopez is racing back into shape - literally. Lopez, 39, is training for her first triathlon with Self magazine editor-in-chief Lucy Danziger - and the singer/actress is sharing her fitness regimen, challenges and all. "You have to start slow and give yourself time," a workout ready Lopez said during an interview from New York's Central Park Monday on Good Morning America. "I'm always challenging myself and seeing what I can do." A self-described "good athlete" who has been following a workout designed by trainer Gunnar Peterson, Lopez says the triathlon presents one big challenge for her."Swimming is a killer, I'm not going to lie" she said, laughing. "[But] I'm a good athlete. My stamina, I can get that up there if I really try." With less than a month until the triathlon, which will take place Sept. 14 in Malibu, Lopez says she's making time for her workouts. And she's not just doing it not just for personal or health reasons: She's raising money for Children's Hospital Los Angeles. Fans can donate money to the cause, follow her progress and read Lopez's blog on Self's official Web site.Named by Self as one of the magazine's 10 most inspiring women of 2008 - and the health magazine's September cover model - Lopez says she is determined to finish the race even if she "has to crawl across that finish line." Her motto? "Get it done, Lopez! Get it done," she said, full of pep. The only downside to the training, though, is the time away from her children, Max and Emme. "It's really tiring," she says during the interview. "I like getting a lot of time with the babies." As a solution, her husband Marc Anthony set up a spot in their home where Lopez can use a treadmill and even work out with a computerized biking program. "I feel great," she said on Good Morning America. "I have more energy - [and] I'm proud of myself."
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