Nothing says "MANLY MAN" like a gym session ... and really hairy arms ... and Mitt Romney is definitely a manly man.

The 65-year-old former Presidential candidate -- along with his wife Ann -- hit a up weight room near San Diego yesterday ... possibly to work off some of that pizza from their "Twilight" date night over the weekend.

Despite losing the election, it looks like having had Paul Ryan as a running mate can still do some good ... for Mitt's abs.