There's still time for Jake Paul to reconsider ... 'cause Mike Tyson just shared footage of his first day of training for their upcoming fight -- and the dude looks like he's still packing a ton of power!!
The 57-year-old boxing legend proved his hands haven't slowed down all that much on Wednesday ... hitting the mitts with his coach Rafael Cordeiro.
Tyson also went shirtless to show off his impressive physique ... saying the fun is just getting started, but it's clear he's taking this fight seriously.