Welcome aboard. I am a "recovering" chronic marathoner, so I get where you're coming from. Mark has some good information about adapting PB for endurance athletes here on the site. Finish the book by all means, but don't skip those resources. They'll enlighten you. I suspect you will find the blog post about Jonas Colting particularly relevant, but here are some others
More on over training
Basically (in case you missed it), it might be possible for you to live Primally and marathon, but it requires compromises and opens the door to possible over training.
That said, Mark's got a great line about marathoningI tell people, if you absolutely decide you need to train for and run a marathon, Iíll let you run two. The first is to finish. The second is to better your time from the first one. If, after that, you havenít broken three hours, itís clear you are not a marathoner. Find another, ďfunnerĒ pursuit.
That line about not breaking three hours needs to be taken with a grain of salt (Mark admitted as much in the comments). Still, consider that, unless you are a rare one like Jonas Colting, it's really difficult to reconcile serious marathon training with Primal living.
Some other suggestions (in no particular order of importance):
Really watch your recovery. You're especially vulnerable to sickness and infections after a long run because your immune system is depressed.
Keep the carbs to an absolute minimum and, preferably, get them from fruit and vegetable sources, not grains (like oatmeal) or refined sugars (like gels and sports drinks).
Ditch the gels and sports drinks in training - they're crap. You can run long without them once your fat metabolism is well developed, so "train low/race high."
After running long be sure to replenish carbs but also consume quality protein and fats with them. The post-run protein will help reduce (but not totally prevent) muscle catabolism.
I'd really prefer that your post-long run breakfast consist of a smoothie made from fresh squeezed OJ, a banana, and some berries (with - maybe - some kefir or plain whole milk yogurt if you tolerate dairy and need the carbs), add some eggs, bacon, and avocado and you're done. Note, though, that if you're not running for at least an hour and a half then chances are you haven't depleted your glycogen to a degree that would require significant concentrated replenishment like juice. Because of the persistent CW nonsense in the running world many endurance athletes grossly overestimate how many carbs even they need. A normal, easy, run shouldn't require replenishment that you can't get from whole fruit.
Consider yourself lucky if you're able to even maintain your lean mass while marathon training. When you make your body run long regularly, it sees muscle (especially fast twitch muscle) as "dead weight." Crossfit will probably help with that as long as you don't overdo it.
Try to keep the running sessions in the "move slowly" intensity zone. Fat's a far better fuel source than carbs and the marathon's a ton more fun when you don't hit the wall.
Let us know how the race goes. Remember: "The second is to beat your time from the first one."