Listen up, Daniel Cormier -- The Miz is handing out some free advice about making the switch to WWE ... telling TMZ Sports the retired UFC champ better be ready to work his ass off!!!
Remember, the MMA legend is a HUGE pro wrestling fan ... and recently revealed he is in talks with Vince McMahon to possibly join the org. as a commentator.
We spoke to The Miz about the potential road ahead for DC ... and he ain't sugarcoating anything.
"It's very difficult," he tells us.