I know before you used to run a loop and set out water. What else are you eating/drinking for the long runs and what is your plan for the marathon?
You should go over the things you have eaten in the last 24 hours or so. Did you have any milk products, raisins, fig newtons, high fiber content foods?
Figure out now what is causing this problem and you should be able to avoid it on race day.
I understand you are too close to the marathon to change things now. I think you have put in a great amount of miles and you should not have a problem getting to your goal. My only question, as I stated above, is what is your fueling plan for race day?
Like I said I have access to a wealth of information about LT training above what you can find online that I would be more than willing to share with you should you decide to go that route for a future marathon.
Like I told Zeke though, I think your next goal should be to get that 5k time way down before attempting another thon.
Keep asking questions. Dan