The bus service is very very do-able.
Yes, the bus is typically full, but they have backups in case several people do not mention it when registering at the front desk.
(At the front desk at check in, they will typically ask if you are participating in the half or marathon, but your 1/2 check in date is ahead of most of the marathon crowds, so you may need to volunteer the information).
They keep a ballpark of how many of their guests will need bus transport. The buses arrive *FREAKING EARLY* at 3:00 - 3:30am, and you need to be on it.
Some people at epcot resorts or Downtown Disney resorts walk to the starting line to get some extra sleep.
When you arrive at the bus, you should have your race number strapped to your chest and some warm clothes. (there is a bag check area to drop off your stuff just prior to race time.)
Hope this helps.
-Tina