Beast Mode ain't playing around about this NFL comeback -- running crazy footwork drills IN THE SAND with a legendary footspeed specialist ... and it's scary good. 

Marshawn Lynch hit the beach with Luis Badillo Jr. -- who's said to have the fastest feet in the world. He's trained world class athletes like Cristiano Ronaldo, Asafa Powell and tons of NFL players. 

This time, he flew up to the Bay Area to put in some work with the Oakland Raiders star -- and it's clear these dudes ain't messin' around.

NFL watch out ... Feet Mode is coming.