The Baltimore marathon is in the middle of October. If you are looking for a smaller, more low-key run that is mostly off pavement and away from traffic, the Northern Central Trail Marathon is always the Saturday of Thanksgiving weekend.
Charm City Run keeps a great Maryland running schedule here - http://www.charmcityrun.com/listofevents
The Baltimore Running Club also keeps a calendar of their events and others at http://brrc.com/brrc2010/BRRC2010Events.html
The Under Armour Baltimore marathon is Baltimore's biggest marathon, and it has become a big success.
This fast out-and-back marathon is held on the Northern Central Trail - there are two miles of paved roads from the start to the trail and from the trail back to the finish (where at the end there is a climb), but this is not a hilly race. The trail is unpaved, crushed stone.
