Very interesting question, Nick, one I've debated in my own head many, many times. I'll share with you a little of my n=1 experience.
I've done marathons "pre-paleo" and marathons since becoming paleo. Since adopting a paleo style of eating, I've competed while eating paleo low-carb, and I've competed eating paleo moderate carb. I found that eating paleo moderate carb is the best way for me. Keep in mind that, whatever you choose, you'll want to stick with a consistent eating style all through training and the race. Switching things up right before or too often will be counterproductive.
I ran Boston 2 years ago. My training involved a weekly long run of around 17 miles, a 5 mile tempo run mid-week, and an easy 7-10 mile run at the end of the week. On the days in-between, I did tabata sprints (warmup+tabata+cooldown totaled about 20 minutes). My weekly mileage was not very high (~ 40 miles) but doing a weekly long run and all the high-intensity work served me well. My diet was 80% paleo, and I didn't go out of my way to carbo-load, but I did eat potatoes and bread. I even fasted for most of my long-run days and didn't suffer any bad effects.
I ran 2:36 that year (we had a headwind that year, not a tailwind like this past Boston!) and had consistent energy throughout the entire race. That was a great experience.
Last year I stuck with a paleo low-carb diet while training for a spring marathon. My weekly mileage was a little higher (~ 60/week). While remaining low-carb, however, my energy for training was inconistent, my long-runs suffered, and my mental energy was really bad. I ran 2:45 for that marathon, by far my slowest marathon.
My diet now actually more resembles WAP than paleo, as I include lots of raw dairy, lots of potatoes, and homemade sourdough bread into my diet. For running, the most important thing I do is one day a week, I run in the morning on an empty stomach. I like to do this for my long runs to help adapt my body to better burn fat at faster paces, so this usually happens early Sunday morning. Once you are fat adapted, you shouldn't need to concern yourself with carb-loading.
I make sure to eat lots of fat and protein, and plenty of sea salt for my adrenals. For eating carbs, I'm usually between 75 and 100 grams a day, adding 10 grams for each mile I put in that particular day. I do think carbohydrate is important for recovery (and also important for the krebs cycle), although some may disagree. If you plan to train a lot for this marathon, a paleo style of eating will serve you well, but don't restrict your carbs too much.
How do you plan to train for this marathon? A 30-minute run is drastically different from completing a marathon, so even if you don't notice a drop in performance there, it doesn't really tell you anything about how you'll feel 20 miles into a marathon.