Hey it's great to see another low carbing marathoner out there! I have posted a few videos on Youtube and blog posts about my past marathons (5 halfs and 2 fulls in the last three years). Prior to my marathons, I eat as close to normal for me as possible -- no new foods, no more carbs, not any more or less food. About the only change is that I might try to eat more chicken fajita salads or stirfrys (lots of protein, fat, and veggies) before hand to encourage GI movement. I also specifically do not eat at least 8 hours before the marathon. I don't need to stock up my tanks as there is enough BF (even at 11 - 13% BF) to fuel a 4 hour run.
Depending on the heat of the marathon, you may need more or less than that, but I drink water as well at every 3 or 4 miles. I end up alternating water (mile 4) - SF electrolyte (mile 7) - water (mile 10) - etc. That strategy works for me, but it does place a dependency on course support to provide custom water stops for you along the course. My wife is gracious enough to do that for me. If you can't get that support, you can try diluting the stuff on the course with water or taking Endurolyte tablets or powder along with you. Given the options, I would probably favor the Endurolytes.
If you really think you need fuel along the way, I think a half banana or even perhaps some nuts could be a boost in energy. I just couldn't imagine the GI issues trying to keep anything down personally, but there is another low carbing marathon QuotidianLight that does.
The reason I focus on the Electrolytes is what happened to me the QC Marathon this year and my training runs. In my training runs without something as simple as Powerade Zero, I dogged at mile 17 or 18. With something every 7 or 8 miles, I am good without issue. At Quad Cities Marathon, I was fantastic until missing my electrolytes at mile 12 or 14 due to my wife getting lost. I think I ran out of sodium around mile 17 again, and never fully recovered. I missed by goal time by 19 minutes even though I PR'ed by 15 minutes.
Well this is getting already pretty long, I'll just post my video links, and feel free to ask anymore questions that I missed.
* Atkins Diet & Exercise: Endurance Athletes & Electrolytes - http://www.youtube.com/watch?v=haWLJh8UC1k
* Atkins Diet: Myth of Carbloading - http://www.youtube.com/watch?v=9GcCZd4Sb_Q
* Atkins Diet: Exercise- It's Non-Negotiable - http://www.youtube.com/watch?v=P1tMIlMYndo