As many have stated, everyone is different. I can do 20 miles easy on an empty stomach but for some people this wouldn't be such a good idea. I'd start with at least a banana or some other lighter fare.
I don't know if this is flawed thinking on my part, but I always feel that if I exercise prior to eating it aids in weight loss as I'm burning what I have instead of what I've just eaten. I also feel like it revs up my matablolism, but again my reckoning isn't anything scientific.