Walking to EPCOT from SSR would be quite a hike indeed! You'd have to walk past OKW, across some very busy roads and then through the parking lots of EPCOT. There are no pathways and Disney security will pick you up and give you a gentle "talking to" if you walk along the roads, which have no sidewalks.
Trust me, you wouldn't want to walk it, even if you could!
For the marathon, you can walk from OKW to the start line for the marathon since it doesn't really start at Epcot but on the road outside of Epcot. But for any other time of the year or any other situation, you cannot walk from OKW or SSR or POR or POFQ to Epcot.