I wouldn't want 2 or more hours in the saddle with running shoes on. Depends upon how serious you want to be - if your going for say 60 miles in 3 hours and battering yourself, then the proper shoes come into their own.
I think the longest I've done is 3:30 in the saddle. And it depends what you mean by serious. If serious means racing (or at least going as fast as possible), then I'm not at all serious. I don't mind doing some fast - for me - rides, but distance is really what interests me.
Runners don't use everyday walking shoes to run in do they?
Barefoot running seems to be the thing at the moment, which doesn't appeal to me at all. I don't fancy getting all kinds of rubbish stuck in my feet.
When I started running, I had terrible knee pain running in a pair of "fashion" trainers, and it wasn't until I had my gait analysed and was fitted for a pair of support shoes that the knee pain stopped. I took to wearing the support shoes for hiking as well, when long walks made my knees ache. I started wearing them for cycling after trying a few pairs of other shoes and getting painful knees, so I'm not so sure that footwear should be sport specific as much as it should be individual person specific. The same shoes seem to work for me regardless of what sport I'm doing.