The 2006 marathon demographics have been summarized and released here. I was a teensy tiny bit disappointed in my Marine Corps Marathon time because (even though it was a big PR) it was very slightly over the median time for the women in my age group in that particular race.
It turns out the average time for women 50-54 in US marathons in 2006 was 5:26:58!! I ran a 5:23:51 at MCM!!
I'm finally (if the statistics hold in 2007) an above average old lady marathoner. :-)