In this link it shows the number of finishing runners based on running time. It may be possible to estimate the number of repeat runners based on performance, but I think you would need someone with more insight into marathons than I have. For example, in 2005 less than 10% of runners finished in under 3:30:00. If this is a good cut-off for a ‘serious’ runner, then you might be able to estimate based on that, but I am not very comfortable with that methodology.
I found this message board question “How many marathons do you run a year?” . It is not a scientific measurement, but I was surprised at the number of runners that do 3-5 per year. However to contrast, many of the marathon events listed do not even make Rumor’s link up-thread. This implies two things: 1) The number of runners that do 2 or more ‘major’ events (i.e. listed in the link above) is very small, 2) There may be a lot of minor events (less than 1000 runners) not accounted for in the link above and the total number of runners could be significantly higher than 464k.