Over at Marathon Guide there is a new interactive marathon map. It's pretty darn neat too! You see a map of the United States (including HI and AK) and marathons are shown as dots on the map. The dots are relative to the size of the marathon, so Chicago is a big green dot. Each of the markers are also active which will take you to the marathonguide.com's page about that marathon. From there you can reach the site's website, if they have one.
Another neat feature is that inaugural marathons are displayed as blue dots, so if you want to be one of the first to run that marathon, you can.
I can tell you that it is not all-inclusive, as the Richmond Marathon does not appear in the time frame of Sept-Dec. Although there are many marathons that are listed and will have a good choice nonetheless. There is a zoom capability, which takes it down to the Main interstate and main city level, this way places where the marathons are in close proximity can be easily identified. Case in point, the MD/DC area has 3 marathons (Marine Corp, Baltimore and North Central Trail (MD)) and in the default view, it's hard to distinguish between Marine Corp and Baltimore, as they are both big marathons. At the most zoomed-in view, you can see all three marathons.
Another feature that you can use, is to see marathons that are run on Saturdays, Sundays/Holidays and ALL Days. There is even a filter to find defunct marathons… not sure why you would want to see them, but maybe someone needs to view. Also, Knoxville Marathon (March) does not show up yet, so it's still in the works.
Here is the description on the site:
We're visual people and we're always wondering: “Just where are those marathons, when are they, and how big are they?” Well, those answers just got easier to find. The newest MarathonGuide.com feature lets you search for marathons with results appearing on a zoomable map interface – it's very cool…