We stay at Epcot resorts - have always had a car, and have just not wanted to mess w/ the traffic situation by driving the car. I have read posts with great advice about driving to the start. If you decide to do that there are plenty who can give you insight on how to time it, etc.
Now I kind of feel like the early morning bus ride is part of the "experience!"

Being with the other participants...everyone anxious and tired and wondering why they signed up for a race that requires a 3am bus ride

(We usually get the bus loading area around 345a.)
We do usually walk thru EPCOT to get back to the resorts (requires park entry so bring your park pass!). If you aren't going to have park passes you will catch the bus back to the resort. Last year after the half we took the bus back, it wasn't horrible but not exactly my favorite memory of the week end, either (cold on the bus, had to wait a while (10-15 min?) for the bus to arrive and then wait for it fill up before we left). I prefer walking back.
If memory serves, it is a bit of a hike from the bus drop off before the race and not as far for the return trip.
One of the mornings we had to wait a while for the bus at the BC. By the time the buses arrived, it took 3 of them to get everyone who had accumulated to the start. I never really get too worried about being stranded at the resort and not making the start bcs there is someone there who calls for buses (I don't know who this person works for - does not appear to be Disney and for sure not the resort).
Probably more than you wanted to know!