Johnny Manziel: 'I'm Doing Really Well' ... Sober and Training Hard (VIDEO)

Johnny Manziel says he's focused on football again -- telling TMZ Sports he's training 5 to 6 days a week and sources say he's "dead serious" about a comeback.

The ex-NFL quarterback was leaving Mastro's Steakhouse in Bev Hills Tuesday night ... and told us how well he's been doing. He also appears sober. We're told it's part of a serious effort to clean himself up.

We know what you're thinking ... we thought it too ... but Johnny is telling people he means business this time around.

Sources connected to the QB tell us he's been working hard in the gym and trying to get himself ready to make another run in the NFL.