The final race results for the 2008 Chicago Marathon have finally been posted (2008 Searchable Race Results), so I can now look at some stats on my performance last weekend. A short aside - the race results posted by the New York Road Runners Club (for example, the 2007 New York City marathon results) are a lot easier to navigate than those posted by active.com.
In any case....
There were 31,335 finishers last week, of which 17,669 were males and 2,768 were males between the ages of 40 and 44. My official time was 4:53:48.
My overall place among the finishers was 17,669 (57.4%)
My overall place among the male finishers was 11,514 (65.2%)
My overall place among the male finishers between the ages of 40 and 44 was 1,887 (68.2%)
In order to place in the top half of my age/gender group, I would have had to finish about 20 minutes faster than I did.
It's possible to get some insight into the impact of the weather on performances by looking back at the stats from the 2006 race, which I believe was run under more favorable (i.e. cooler) conditions. A time of 4:53:48 would have been closer to the bottom performances of the Male/40-44 age range (84% rather than 68%), and placing in the top half of the Male/40-44 age group would have required a time of 4:01:47 - a full 32 minutes faster than the median time this year.