I was actually probably reading the same Science of Sport posts Chad was referring to. That's a great blog...highly recommend it.
This ought to get you started: http://www.sportsscientists.com/search/label/hyponatremia
I think in all aspects of long distance running, whether it is how much to train, how hard to train, how much to drink, how [insert almost anything running related here], we have to learn how our body reacts to a given practice, and what makes us "tick" the best. Sure, there are general principles that can help almost everyone, but how we apply them is, again, a very individual thing, and it's sometimes tricky to learn how to determine how we apply the techniques we learn.
Additionally, in reference to Colby's comments, it's better to do most of your training at an "easy" pace. You definitely do need to have workouts that focus on the "speed" aspect of things (and again, how much you need is largely individual), but the bulk of your miles need not be at anything more than an "easy" pace. This relates to your gastric issues because higher intensity always has a bigger effect on the state of your GI. I usually don't have problems, but on the rare occasion that I do, it's almost always caused in part by high intensity, combined with perhaps improper (usually too much in my case) hydration and/or fueling. If you're training for the marathon, some general principles on how to approach the marathon training-wise, which have helped a lot of people on this blog, can be found here: http://fastrunningblog.com/forum/index.php/topic,1076.0.html