Adrian Peterson Begins Training For Boxing Match With Poker Rival

Adrian Peterson's begun training for his celebrity boxing match with his poker rival ... and while his hands could certainly use some more sharpening, it's clear he's in great shape.

Check out some videos TMZ Sports has obtained of the Minnesota Vikings legend as he kicked off his preparation for his big matchup with Joe "Baby Joe" Castaneda ... he looked like he could still pick up a few first downs for an NFL team.

Rockin' a maroon tee, grey shorts and black shoes ... the 40-year-old showed off a svelte frame as he hit the mitts in a Houston gym.

The former tailback's fists weren't as quick as he probably would have liked -- but he was moving extremely well, and it sure seems like he's taking his fight vs. Castaneda seriously.

There's still no time, date or place set yet for the scrap -- though Celebrity Boxing founder Damon Feldman said he's working on getting that all secured shortly.

As for Peterson's opponent, Castaneda has also already begun his prep for the match ... as video we got earlier this month showed he, too, has been working with a trainer to make sure he can land a win whenever the fight goes down.

It'll all, of course, be a rematch from May 27 ... when the two dudes waled on each other following a dispute over cards.