It seems to me like there are not as many marathon runners today when compared to the 1980s. There seems to be more marathon races today so that may diminish the fields for some races. Today there seems to be as many or more people at the start but only half are actually running the complete marathon. The others are running the half or the relay or the 5k. In some of the smaller marathons I've entered, everyone starts at the same time: the marathon, the half-marathon, the 5k, the two-person relay, and the four-person relay. I think that most people have run a couple of marathons and found that they aren't lowering their times and so why invest all that training time when they can be in just as good physical condition by training for shorter races. This way training is more enjoyable and there is much less chance of injury.