Also I know for special events on the Disney property that the Fort runs the internal busses as early as needed to accomodate guests. I stayed at the Fort in different years when I did the Dis Half Marathon and Full Marathon; each year the internal busses were running at 4a.m. which was the time I left. They had busses running over to EPCOT that time of morning too.
So I don't know the relevant "show up" time for the triathalon but I can assure you the busses will run early enough to get you there on time.
I agree with pp. We were there last year during the Ironman. I know that it was posted that the busses would be running EARLY. Don't remember what time they started, and I'm unsure as to how often they ran.