The armchair approach would be the easiest. If the route can be followed on Google Earth the horizontal length is easily obtained.
As Mark says, the elevation will likely make little difference. However, if, as you are mouse-measuring on Google Earth, you record the elevations (at bottom of screen) as you go, you can correct for elevation. You will have a series of straight-line measurements, each bracketed by an elevation.
Geometry will do the rest.
This is likely bad advice, as I'd be surprised if the route is easily determined using Google Earth.
How about posting before the race:
The course does not lend itself to dead accurate measurement. Over the years the distance has become accepted as the about standard marathon distance.
Although personal GPS devices will have some error, we will record, post-race, as many readings as can be gotten from the year’s participants and publish those results.