Offhand, I wouldn't really see "marathon" as a big terrorist magnet either.
Oshkosh could be easily targeted (no metal detectors, can enter without a search at all if one flies in, a membership base that'll resist most security measures) but it doesn't really offer a concentration of people to target. There are a lot of people there, but even during the airshow, they're spread over a mile or so of flight line. The finish line of a race is more likely to have a concentration of people...
Ron Wanttaja