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Miley Cyrus knows how to warm-up a crowd.

The singer made a surprise appearance earlier today at the 13 Annual LA County Walk to Defeat ALS.

Cyrus popped up on stage to support Mari Winsor, a celebrity Pilates instructor who is battling the disease, as she lead a warm-up session for the event's 2,000 participants.

"I'm already sweating," Cyrus told me as she walked off stage. "She had us doing a happy warm-up and she kicked my ass once again. She's done it for years and she's already done it this morning and it's only 11!"

Miley Cyrus

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The day's top fundraising group was Team Nanci in honor of longtime Hollywood power publicist Nanci Ryder.

"I'm just happy to be here supporting Nanci but also just raising awareness and raising money for ALS," said Reese Witherspoon, who participated along her talent agent husband Jim Toth and her 16-year-old mini-me daughter Ava Phillippe. "There's so much that's not known about this disease and watching our friend progress every single week with something different and not know is so horrible.

"There are so many families living with this devastating disease," the Oscar winner continued. "It's time that we start doing something to create a cure, to find what the root of the problem is and eradicate it because it's just unbelievably devastating."

Other celebs on Team Nanci included Renée Zellweger with musician boyfriend Doyle Bramhall, Courteney Cox with daughter Coco Arquette, Angie Harmon, Justine Bateman and Daphne Zuniga. Some of Hollywood's most powerful executives also wearing "Team Nanci" t-shirts included DreamWorks Animation boss Jeffrey Katzenberg and talent agent Bryan Lourd of CAA.

For more information about ALS, go to asla.org.

Courteney Cox, Reese Witherspoon, Nanci Ryder, Renee Zellweger

Frazer Harrison/Getty Images