COMING FOR YOU, PETERSON!!!
Joe Castaneda has already put hands on Adrian Peterson, but "Baby Joe" isn't taking the NFL legend lightly ahead of their Celebrity Boxing match ... he's started working out with a trainer, and TMZ Sports has the video!
We're told Castaneda -- a professional poker player by trade -- hired combat sports coach Teddy Concepcion at the Oasis gym in Las Vegas ... and Joe is already on the ground, going through a bunch of different drills, including hitting mitts.

GETTING IN THE RING?