PO-Riverside is a host hotel, so you're covered there.
I googled and found last year's instructions. This year's haven't been released yet. Here is the link to last years....
http://adisneyworldsports.disney.go.com/media/dwws_v0222/en_US/media/amateurSports/PDF/Endurance/RoyalFamily5KFRI2010.pdf
Here is what it says about transportation...
PARKING & TRANSPORTATION
"There will be bus transportation provided for this event if you are staying
at one of the eight host Walt Disney World® Resorts. Transportation
will run from 5:00 a.m. – 6:00 a.m. prior to the race. The last bus
from the host resorts to Epcot® will depart at 6:00 a.m. We ask that
everyone please arrive at Epcot by 6:15 a.m. Bus transportation is
suspended from 6:30 a.m. - 7:15 a.m. due to race activities. Buses
will then run from 7:15 a.m - 9:00 a.m. after the race to return you to
your resort. There is no monorail service to the start of this race!"
Wouldn't expect it to be much/any different this year, but who knows.
Maura
