You can not walk to the start from Board walk or Yacht&Beach. You take a bus, EPCOT will not be open for you to walk through it.
Also a note, if you plan to walk back through EPCOT after the race to one of those hotels - YOU NEED YOUR PARK TICKET. Otherwise, just take the bus back. Any hotel is good and the chartered buses operate very well. I do like to stay at the POLY, now don't get me wrong, I am a POP century Girl. But this past year I stayed at the POLY as a surprise to my Mom and LOVED IT. Liked getting on the Monorail in the AM and heading over to EPCOT. Easy for my cheering squad to walk over to TTC and then monorail to MK and then monorail back to EPCOT to see me finish during the Half Marathon.
Just my two cents...
Tracy