Koya-san is magnificent, IMO, but best explored before and after an overnight temple stay. And I don't think it is just 40 minutes from Hiroshima -- I think it's substantially more than that, but I could be wrong.
http://www.japan-guide.com/e/e4900.html

Of course it is more than 40 minutes from Hiroshima. A cursory look at the map will make that plain, and consulting hyperdia.com will provide the actual time by rail.

Ah, it is just under four hours from Hiroshima to Gokurakubashi, which is where you get the funicular that connects to the bus for Koya-san.

It does help to check your data before posting.

(Last time I checked before posting, I inadvertently entered the wrong START time into hyperdia.com, and ended up posting a markedly discrepant time for traveling from place to place. I'm a bit reluctant to enter times again....)

(My comment was intended for bussa17, who was way, way off.)

(Understood - thanks, thursdaysd!)

Thanks again for all your advice