I think the main issue here is that there is a big difference in what people are calling "marathon training". The women I see out in my training group who are all very fit and lean (but not elite athlete lean) all train very hard for marathons. They run 40-50 mpw, maxing out @60. They push themselves very hard at interval sessions, tempos and long runs. On easy days, they still run with some pace and do not just shuffle out 12 min miles. No one obsesses about diet other than making the usual good choices on what to eat.
Then there are the people who go out with the "couch to marathon" programs and run minimal mileage, very easy workouts and most everything is at a pace that is just faster than walking. On long runs, they will stop for 10-15 minutes to chat with friends. The pace is so slow that they can talk the entire way and sound like they are sitting on the couch at a coffee shop. At first, the increased activity might knock off a few pound. But after awhile, they acclimate to the distance and consume enough calories to easily cover what little was burned off. If self control goes out the window, they will gain weight very quickly. But the result would be the same if they were doing yoga, cross fit, pilates or cycling. They just think that because they are doing a marathon "training" program they can eat freely. If they actually trained for a marathon instead of doing the minimum amount of shuffling needed to finish the race, they would not be gaining weight.