Just HOURS after crushing the KC Chiefs' Super Bowl dreams ... 38-year-old James Harrison was back in the gym -- getting ready for New England.
The guy is an absolute maniac ... busting out speed drills, power squats and back work.
By the way, Steelers coach Mike Tomlin (in a Facebook video streamed from the locker room) had estimated the Steelers jet wouldn't even land until 4 AM!!!!
#BEAST #BEAST #BEAST