Riding the bus up to my first marathon, some wiry, skinny older guy was sitting next to me, and said something to the effect of how so and so famous runner started a marathon recently but couldn't finish, and "that's what makes this so much fun. You have no idea if you'll be able to finish once you start!"

A little bit fatalistic and depressing (especially to a first timer), but true. Running is unpredictable. IF your body, mind, nutrition, etc. aren't right, anything can happen.