To compare marathon running, which is a performance-based sporting endeavor, with bodybuilding, which is a pageant, doesn't really work. Attending a bodybuilding pageant would make most people pretty uncomfortable. Shaved and oiled nearly nude grown men prancing around a stage to cheers...if you take a step back it is obvious how bizarre the spectacle is. Seeing men in such an effeminate position is uncomfortable for a lot of people...the way many straight people would be uncomfortable at a gay bar or a drag show.
On the other hand, any kind of respectable performance in one of these pageants takes a lot of natural ability, a lot of discipline over an extended period of time, and years of hard work. Pretty much any schlub, barring serious obesity or other health issues, can train for and complete a marathon in a few months' time, then go back to being a schlub.
As far as just building the body without any competitve bodybuilding aspirations, I'd say there are at least an equal number of people involved in that pursuit as there are in endurance sports. Or at least it's close.
And I will add that for a competitive bodybuilder, distance running probably doesn't do much that can't be accomplished with less intrusive forms of cardio, like a treadmill or elliptical or whatever. Schwarzenegger used to jog a good bit, though, didn't he?