Regarding weight gain while marathon training

I am currently training for my 4th marathon, and I always gain 5 to 10 pounds of weight as well before each marathon. Although I do lose the weight again after the marathon, I dislike the feeling of that extra weight on me, and always wondered what the cause was. I did cut out a lot of my cross-training activities and replaced them for training runs which I thought might be the culprit, because as far as caloric intake, I've kept a daily diary for years, and I had not been consuming enough extra calories to account for all of the pre-marathon weight gain. I did think that some of the weight might have been water and extra muscle weight, but was still convinced that those two factors did not account for all of the extra weight that I always put on.

However, a month or so ago, I received a regional paper put out by a running organization in the Washington DC area, and there was an interesting article about marathon weight gain in it: The author of the article was a specialist in metabolic counseling and testing (I can't recall if she was a nutritionist by trade or something similar). She wrote about a woman who came to her also complaining of a 10-pound weight gain while marathon training. The author tested this woman's metabolic rate via blood and exercise tests. What she found out was that the woman's ideal fat burning range (was it maybe around 120) was well below the heart rate that the woman ran/marathon trained at. Therefore, the woman was doing all of her running and exercise work in the cardio range, and not burning fat. The author encouraged the woman to add 3 weekly 1-hour walking sessions to her exercise routine, and the woman lost the weight shortly thereafter.
Now, when I first read this article, I didn't agree with the conclusion because common sense tells you that calories in=calories out, and that this equation could not be altered by other factors such as heart rate while exercising or exercise type. However, although I did not have a metabolic test performed on me, I did decide to try the addition of low heart rate activities to my training routine, like the author had suggested. I have lost several pounds since adding low heart rate activities such as walking and biking to my marathon training schedule.
Sorry for the long post, but I just wanted to get that out there for those of you who seem to be suffering from the same pre-marathon weight gain phenomenon. I would love to hear comments to this theory - I still am not completely convinced that I'm right!

-a post on the Runners World forums, reminder to myself to try this out.