Welcome! Your first marathon is very exciting!
Fueling is critical for the marathon, and while there will be a bit of fuel on course (usually bananas and Sport Beans, as others have mentioned), it's definitely not enough for the whole race. Use your long runs to figure out what works for you. It's worth tweaking your vest/belt setup to try to find something that works for you even just because that will help a lot with long runs. I can't imagine running 18 miles without having something to carry my hydration and fuel!
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@FFigawi, I run longer races with a Naked belt - if you get the right size, it's really not noticeable. I can fit two 8oz soft flasks, quite a few gels (I aim for 70-80g of carbs/hour), and my phone in there with no problems. I know people also like Flipbelts for similar reasons, though I've found they bounce a bit more and don't hold as much. Flipbelt makes shorts with a lot of pockets as well if you really can't do a belt. I haven't tried those myself, but I have heard numerous people recommend them.
If you're traveling with someone, there's also the option to have them meet you at some point on the course to hand you additional fuel. There's a small risk that you won't find each other for whatever reason, but other people have successfully done this in the past.