It all comes down to the attitude. I hesitate to come down on a person I don't know whose name is attached to a six-hour marathon, but if I know the person and I know they're only doing it as a lark (i.e. not training seriously) I have no problem telling them they're not a "real" marathoner.
I could go out and run a marathon in 2:45ish right now, but I wouldn't consider it an accomplishment (and would never do it) because I would just be going out and doing a longer-than-usual long run,at a fairly comfortable pace. That's infinitely worse (IMO) than someone like my dad who would train his ass off for a year, run himself to exhaustion in the race, and probably finish around 4:30.