Travis Kelce's tackling his offseason dad bod head-on ... by taking some solo time away from Taylor Swift, and getting back into the gym with his personal trainer.

TMZ has learned Trav is back training with Laurence Ng -- his longtime trainer and owner of Legendary Performance in Bev Hills -- and if the workout guru's latest Instagram video of TK in action is any indication, it's clear he's already got his head back in the fitness game.

In the vid you can see Travis in an intense training session with Laurence and they ain't skipping leg day! Whether it's on a treadmill, or sprinting up a parking structure ramp ... he's all about running drills to stay fit for the next NFL season.

It's clear Trav is working overtime to keep up his stamina, and get back in shape after he admittedly carried a few extra lbs. during his recent Bahamas getaway with Taylor.

He's also proving even though he snagged that Super Bowl win with the Kansas City Chiefs this year, the grind never stops.

Clearly, Laurence has cracked the code when it comes to Kelce's workouts ... after all, they've been at it together for several years!

TK's sticking to his tried-and-tested routine, 'cause as we told you last week, contrary to reports, Travis doesn't hit up West Hollywood's Dogpound gym with Tay Tay.