8/17/2012 10:00 AM PDT

Here it is ... the very first photo of Paul Ryan's mysterious athletic physique ... REVEALED!!!

TMZ has (finally) obtained a photo of Mitt Romney's running mate -- sans shirt -- taken roughly 6 years ago ... right before Ryan got hooked on the P90X workout.

We're told the pic was taken in Oklahoma during a family vacation -- the woman on the left is Paul's wife Janna.

While Ryan's bod ain't bad in the pic ... sources close to Paul's abs tell us Ryan has totally transformed his midsection in the past couple of years ... and now he's totally shredded with a killer 6-pack.

 We spoke with someone close to Ryan's family who tells us Paul has "always led a healthy lifestyle" ... adding, "He's the nicest guy and has always been fit."