Credit: Matthew Symons/Coleman-Rayner

Work it out, Kourtney!

Kourtney Kardashian was looking more fit and toned than ever in micro-mini shorts and a sports bra top in Malibu on Thursday.

In photos exclusive to, the reality star and mom, 31, showed off her gym-perfected bod -- jogging and doing step-ups beachside -- as she shot pics for a fitness book.

The Kourtney and Kim Take New York star welcomed Mason Dash Disick, her son with beau Scott Disick, in December 2009 -- and stunned fans with her incredible post-baby bod the following summer.

How'd she do it, besides those workouts?

"Definitely breastfeeding helps," she told Us last year. "Everyone says that, but it's really true -- and it's just being a busy mom."

And motherhood never stopped her from feeling sexy, she told Us. "It brings out a whole new glow or something!"