I would say that running marathons these days is pretty popular. I venture to guess that there are 1) more marathons organized in 2008 than in any year in either the 70s or 80s (how about any 2 years) and 2) there are more people running marathons than in any year during that era.
However, little to none of that seems to cross over into people being fans of track and field.
I can't help but wonder if the surge in the number of marathons is simply due to it being a money-making business. I don't know the financial bottom line for marathons, but it seems like many of them should be doing quite well with such high entrance fees and big numbers.
I also think that's why so many marathons nowadays offer races of so many distances (often 5K, half, and full) on the same day because they can make more money using the same road permits with more people out there.
But the net effect, at least as I see it, is that all these new marathons are vastly watering down the competition level in the older ones.