Although, there are different interpretations one can decide to give the word ''fit'', but looking at the subject of this question. The word 'fit' is actually referring to the shape and physical stability of an individual. Arnold Schwarzenegger was once a bodybuilder, though he might still be doing that. As a bodybuilder, he has won so many awards, and he has indeed achieved a lot, that's why you will see people referring to him as the greatest of all time as far as bodybuilding is mentioned.
If actually, we are to base our judgment about the fitness of Arnold Schwarzenegger during his prime on his physique, then there is no other word to describe him than the word 'fit.' The truth is Arnold Schwarzenegger has a perfect muscular structure, and he's a fan of anything that pertains to all these cardiovascular exercises. He likes gyming, swimming, jogging, bicycle riding, etc.