Kim Kardashian -- My MILF Secrets ... Exposed!

Kim Kardashian's great ass and post-preg toned body didn't come naturally ... and last night in Beverly Hills she told us her secrets to being a full-blown MILF.

Kim was leaving La Scala with Brittny Gastineau when we asked about the Instagram photos she and Blac Chyna posted hours before ... showing off their hot mom butts after a workout sesh.

A humble Kim had no problem telling us how she got super fit.

Hint: She didn't say photoshop.