short and sweet and I think they can get away with another series quite easily. How long running was the original manga?
Ongoing. There's the webcomic, which is the true original version, and then Yusuke Murata (who previously drew Eyeshield 21) is redrawing the webcomic "professionally".
There's basically one full arc after the events of the anime, with a few little mini arcs/stories, but it's a really long arc. The anime covered about chapters 1-40 of the webcomic, and the new arc takes about 41-95.
The thing with OPM is that progress is slow, because the webcomic author also draws a weekly serialised manga called Mob Psycho 100 (which is also getting an anime release soon), so OPM is basically updated whenever he can.
I personally love it, and don't think it gets boring or repetitive. The thing is he's made a world with all these other heroes. Of course if it were just Saitama, it would get boring because he's clearly overpowered, but really Saitama becomes a bit more of a trump card while we see the other heroes in action.