Pain on the bottom of my foot...

OK this might not be it but I had an old teammate who had a very similar pain.  He was having very difficult recovery problems that would affect him for hours, and eventually days.  He finally went to the doctor and found out that he had a VERY small stress fracture in one of his foot bones, and after long workouts it would become inflamed and thus the pain.  He was out for about 8 weeks so that it could heal.

 I, of course, don't know if you have a fracture, and you're probably too scared to go to the Dr. for fear of him telling you that you can't compete!  Hell, I call tell you all day, "you've got to go to the doctor!!!!" but I know what that feels like.  But if you DO have a fracture and you injure it in the race, you could be out for months.  You might want to have it x-ray'd to be sure it's not something really evil. Worse case scenario you knnow how much Tylenol it will take to get through recovery.

Other than that I could only think of things like needing new shoes or too much heel-striking or needing to run/walk.  I hope it works out!!!!