Interestingly, marathoning in America has reverted back to a Century of the Common Man mode rather than follow the rest of America into the Century of the Superstar, where a few experts participate but many watch. I can't recall a famous American marathoner since Alberto Salazar and Bill Rodgers many years ago. And yet, despite a complete lack of famous role models these days, as a social movement in America, running marathons is quite healthy these days.
A friend wants to find demographic data on who pays to run a marathon (not the professionals who finish in the top ten, but the amateurs who run to see how well they can do or just to see if they can finish). A Google search on "marathon demographics" brings up detailed data on sex and age, as well as a few marketing research reports saying sponsoring a marathon is a good move for a financial institution (like LaSalle bank, which sponsors the huge Chicago marathon) because marathoners are just like golfers, only younger, more urban, more energetic, and even more into delayed gratification. But I can't find anything on the race or ethnicity of participants / finishers.