With the increased training leading up to your race, it is possible to have developed a stress, or fatigue, fracture. From where you are having the pain it sounds like it may be in one of the metatarsals. On the other hand, this may simply be inflammation in the tendons and muscles also arising from your training.
At this point, I would recommend several days rest, ice for twenty minutes several times a day, elevation if needed, and over the counter NSAID's if you can take them. If the pain persists for more than several days, see your doctor for an examination and an Xray.