Does carb-loading before a marathon work?
Yes, eat extra carbs for about two days so you don't hit the wall.
To run your best marathon, go ahead and eat a lot of carbs. Like, a ton of carbs.
Start by eating about 5-10 grams of carbs per kg of body weight each of the two days before the race. As a benchmark, that's about 340 grams for a 150 pound person at the low end—or over 8 servings of spaghetti. So plan ahead and add carbs throughout your meals and snacks.
The morning of your race, top that up with a carb-rich breakfast. If you didn't fully carb-load in advance, you can partially make up for it by eating about 200-300 grams of carbs a few hours before setting out.
Why so many carbs?
The goal is to delay "hitting the wall" or bonking. Glycogen reserves (or stored carbs) are the predominant source of fuel for strenuous exercise. By increasing carb consumption and reducing activity for a couple days, trained runners can build up enough glycogen to fuel about 90 minutes of moderate to high effort. Once that glycogen is depleted, muscles have to rely on carbs consumed during the race and stored fat, which are less efficient. So load up and you'll run longer without slowing down.