I'm with you. I'm due to start the day before my big race too. Happened last year and the time before. I'm not sure it would help you this late in the game, but you might try something like Midol that helps reduce water retention and also ease cramps with Tylenol, which is OK to take prior and during the run. I know you should avoid salt but it's important to get enough sodium during a marathon, so chuck that advice.
My big problem is flow. It's bad the first two days and requires me to make more bathroom stops than I'd like. Have yet to find a product that truly works on the run, in that regard. I wear a super OB tampon AND thick pad, but inevitably both need to be changed at least twice before the finish. Just a fact of life, I guess.