Jennifer Lopez's triathlon efforts for charity

By Stone Martindale Aug 18, 2008, 19:27 GMT

07/03/2008 - Jennifer Lopez - Jennifer Lopez Arrives at Mamamuy Childrens\' Store in Barcelona on July 3, 2008 - Mamamuy - Barcelona © Solarpix / PR Photos

Childbirth is not stopping Jennifer Lopez from racing back into fighting form.

Lopez, 39, will attempt her first triathlon with Self magazine editor-in-chief Lucy Danziger – and J-Lo reveals her fitness regimen, challenges and all inside the magazine.

"You have to start slow and give yourself time," Lopez said on Good Morning America. "I'm always challenging myself and seeing what I can do."

The actress and singer has been using trainers for some time, but this takes her fitness to a whole new level.

"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."

The triathlon is scheduled for Sept. 14 in Malibu, California.

A philanthropic Lopez is also raising money for Children's Hospital Los Angeles by her fitness efforts.

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 – Lopez says she is determined to finisheven if she "has to crawl across that finish line."

Her rigorous training does cut into her mommy time with twins Max and Emme. "It's really tiring," she says during the interview. "I like getting a lot of time with the babies."

"I feel great," she said on Good Morning America. "I have more energy – [and] I'm proud of myself."