The first result one reads is the steady increase in marathons finishers: from the few thousands of the mid-70s to the half a million of the past year. This phenomenon has prompted many to call for the first running boom since the 70s. Some have linked this renaissance of running to the current economic recession: the sport is fairly cheap and doesn’t require gym memberships, furthermore being active helps to feel better. Although this looks sound, it doesn’t fully address the observed phenomenon, since the numbers are from races and marathon fees can be as high as 150$: it doesn’t look like people laid-off could easily afford those races. Maybe the increase is not (mainly) among the unemployed 9%, but (mostly) among those who are feeling the stress of an uncertain future and then consider a race as a motivation to keep going.
The other result worth to mention is the shortening of gender disparity among runners: the image of Kathrine Switzer  being chased by Boston Marathon officials because she had the impudence to defy the ban on women seems to be a thing of the past, finally.
The field is not only welcoming more women, but it is also more open to the masters : from being 25% in the 80s, they are now comprising almost half the field.
The present recession may have something to do with this too, but I am wondering whether health concerns may have been a bigger and stronger stimulus, in fact 75% of the RunningUSA survey consider ‘staying healthy’ as a major motivation to continue to run.
The drawback of deepening the field is the constant slowing of the median times across all major US marathons. This has led many, on running forums and blogs, to constantly complain about the “penguins,” that is, the slow back-of-the-pack. Personally I don’t care: so far I’ve never been negatively affected by slower runners and I’ve always been able to run my own race. But it has become a highly combustible argument, since the growth of the running community has the unwanted consequence to make races fill up really quickly, sometimes absurdly so, like last year Boston.
Speaking of Boston, this race is still one of the fastest in the country, despite the tough course but thanks to the qualification process. It is most striking when compared with New York City and Chicago, which with Boston (and London, Berlin in Europe) are making up the World Marathon Majors
The chart shows the percentage of sub-4 hours finishers in Boston, New York City, and Chicago: this data set can be considered a good estimate of how fast a race is, in terms of performance. From 2006 till 2010, two-thirds of Boston runners are finishing in less than 4 hours, while in New York City “only” one-third. Both races are considered technical (as in tough), so in many ways they are comparable, but of course NYC doesn’t have qualifying standards like Boston and so its field consists of runners of a broader range of abilities . The Chicago data set is also very interesting: it’s all over the place, ranging from 14% to 35%. I doubt that the field could have changed that dramatically over one year, so the culprit must be somewhere else and I think it’s the weather: because of the unpredictability of its weather (largely its temperatures), Chicago is a hit-or-miss marathon. (I wish I had known it before.)
The RunningUSA 2011 Report shows many more trends, from the incredible growth of the Half-Marathon, which is becoming the race of choice among most recreational runners, to the demographic of US runners: it’s an engrossing read if you love both running and data.
 The story of Kathrine Switzer and her 1967 Boston Marathon has been told over and over as one of the most beautiful example against discrimination; for more I can only link to Switzer’s website. [back up]
 This is not entirely true: both Boston and NYC have qualifying standards, but for Boston they comprise the majority of the field, while for NYC most runners enter either with the 9+1 rule or through lottery. [back up]