I don't think Lucas knew either. Watch a video called "How Star Wars was saved in the edit" on youtube to see what a mess he made and how the editors (his wife among them) cut the boring crap, made up new stuff, changed the order of everything, etc. And made it good.
editing can do a lot for a film. Supposedly a test screening (opening?) of 2001 went over poorly (although even today there are widely mixes reactions) and Kubrick went out to Sam Goody or something and bought some classical music to rescore the film.