on most road rides and centuries (gosh, can't believe I'm saying that, till last year the longest I'd done were metrics) I can deal with 20 miles between rest stops. Must admit that I think it was day 5 of the AIDS Lifecyle that on the central coast I and a bunch of other riders created our own rest stop behind some trees, we were out in the middle of nowhere and I knew I was never going to make it, wasn't even a town for 15-18 miles.

Also the weather is key, if it's hot and or you're working real hard you're going to sweat more of it out.

But don't forget the bike, if your a roadie and your bike is beating you up that can be it. On ALC4 my ex (at that time current) said that she ALWAYS felt she had to go. She was riding a Cannondale that was so stiff she felt every gum wrapper it went over. That bike was super fast but she said from day 2 her kidneys hurt. I understand since then she's gotten steel. So if you've ruled out medical reasons I'd start thinking comfort.