Amber, this is a tough question! You might want to try posting in the exercise forum and see if someone there can tell you if the carbs in SBD will be enough to last you through marathoning...I don't know enough to say.
That said, personally, I'd consider trying to do a modified Phase 2 until the marathon is over. That's what I'd call 'setting yourself up for success.'
So...here's what you'd do:
Follow the book for the Foods to Enjoy list for Phase 2...be aware that since you aren't doing P1 first (do it when the marathon is over, okay?) you'll probably have cravings, so you'll have to try to have extra control over how much you eat, okay? You'll want to take a look at the lists for P2 in our FAQ to make sure you have the most up-to-date stuff, since the book is often behind, especially if it's a hardback version. As to what you mentioned eating, here's the P2 substitutes:
Whole wheat pasta the night before
Morning of: whole wheat bagel (check ingredients very carefully...make sure there's nothing that says 'enriched') and a medium banana
During the run: SBD protein bars, whole wheat pretzels (you might find it easier to eat Newman's Own Organic Spelt pretzels...they also have a bit of protein, which is helpful), and no starburst. If you need a fast jolt of energy, instead of the starburst, consider a small piece of fruit...maybe grapes? Or small pieces of dried fruit?
Hope this helps. Good luck with the marathon...what an amazing thing to do!