Training camp doesn't officially kick off until next week, but Aaron Rodgers is getting an early start on things ... hitting a popular L.A. gym for a marathon workout sesh this week!

TMZ Sports spotted the 42-year-old future first-ballot Hall of Famer leaving a Gold's Gym location in the City of Angels.

Rodgers spent about 3 hours inside the facility, which is regularly visited by some of the biggest celebs and athletes in the city.

On his way out, we tried to talk to him, but he wasn't in a chatty mood.

The 4x NFL MVP has been busy lately.

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He recently revealed he patched things up with his parents after years of being estranged, posting photos hanging out with his mom, dad, and brother Luke.

Summer break is about to come to an end for AR, who will soon report to Pittsburgh Steelers training camp for his 22nd and last season in the NFL.

One last camp, one last season -- and he's not waiting around to get ready.