Pippa Middleton's Waist-Whittling Workout Routine

Catch her if you can! Duchess Kate's younger sis never slows down!

Two weeks after finishing a June triathlon in Woodstock, England, Pippa Middleton competed in Scotland's Highland Cross race, a 20-mile run and 30-mile bike ride.

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"She does it all: tennis, swimming and running," an insider tells Us Weekly.

But the 5-foot-9 single stunner's true love? Hourlong Pilates sessions with Margot Campbell at London's Pilates on the Go.

"I always leave feeling calm," Pippa, 28, has said of the mat- and Reformer-based workouts. "I have noticed a huge difference in my core strength."

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And, of course, the sessions help form her famous body part. Reveals a pal, "The kneeling one-legged pull on the Reformer gives her bottom that pert and round shape. She takes her workouts seriously."

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While it's clear that Middleton, who is the author of a forthcoming party-planning book, loves her daily caffeine fix (specifically at London's Le Pain Quotidien) to get her metabolism revving , she also "loves fish and Mediterranean food," says a source.