We stayed at BWI and they share a bus on marathon morning w/ BWV. I don't know exactly how frequently they come around but our experience last year was good. We walked outside to the bus stop and a line of runners was formed waiting for the bus. Cast members were at the bus stop w/ phones in hand managing the situation. We waited less than 5 min for a bus and we got out there about 320a-330a. I remember seeing mostly the cruise ship buses in use that morning, not the disney transportation buses that you use around the property. I am thinking we might wait a bit longer this year to catch the bus, probably 345a.
We took our park hoppers w/ us in our fanny packs and walked back thru EPCOT after the race to go back to the resort. It was great--got to cheer on more runners/walkers that were still on the course and also got rid of some lactic acid. (We cut thru by Canada/England and went out the International Gateway onto the Boardwalk)