various suggestions wrote:
1) not eating grains helps a lot;
2) drink water at night instead of eating;
3) drink a quart to half gallon of water 30 to 60 minutes before running;
4) have a pit stop arranged about a mile from the start.
Starving yourself then drinking a HALF GALLON(!) of water before your run is going to lead to as many problems as having to poop! You might not have to poop but you'll be stopping at every tree to take a leak.
As others have suggested, use something to stimulate your bowels and give yourself more time to spend on the can. A cup of strong black coffee followed by 30-45 minutes on the toilet should take care of everything. If you just sit there you'll usually get hit a second time by the gingerbread man (any marathon talk listeners?). Bring your iPad or a book so you have something to do.
If all else fails, add nicotine to the equation. Coffee and a cigarette or a lip of dip is like pouring Draino into your system.