See my post below under the thread called Leaky Bladder dated 8/4/03. I ran several marathons this year with no problems. But if that doesnt work for you, maybe your doctor can prescribe a custom device that will do the same thing.
As for staying hydrated on long runs, its kind of tricky because removing/replacing the device is such an inconvenience. On my long training runs, I always carry my own water with me so I know how much Im drinking under a given set of environmental conditions. I break up my route into 2-3 out-back segments where I know Ill have the opportunity to get more water and make a bathroom stop, if needed. I practice drinking enough to run strong but not so much that Id have to use actually the bathroom. By race day, Ive got a very good idea on how much fluids I need and how often.
Good luck and I hope you won't let this problem stop you from completing your goal!