There is a product called a stool softener where you sprinkle some powder on food or mix it in a drink. I'm not sure what the brand name is. I used it once when I had severe constipation. It was such a bad feeling. The product worked super. It sounds like if you used something like this the night before or two nights before it may help you. Also for some reason certain foods make me have a major bowel movement the next day. Pancakes and vegetables like corn and lima beans especially. Pancakes for some reason give me energy but it seems like I 'dump' more than I eat. I guess my body doesn't process it too well. Certain foods like chicken and spagetti give me energy without the severe bowel movement. You might try experimenting with different foods. It may take months but you may be able to find the right foods for you to eat during the day preceding your race. Don't feel bad about it. I'm sure many people besides me have this problem but may not like to talk about it because it is kind of embarassing.