I ran WDW marathon in 2000 so my memory may be incorrect but there were buses going to WOS from POP. Whether those were special shuttle buses designated for the marathon registration or the regular WDW buses I cannot recall but there will be buses available regardless. My recollection is that it is not within a reasonable walking distance, particularily if you are trying to rest in order to run the marathon or the half.
Good luck. It is a fun, flat and fast course. Altho you will discover how small the parks actually are as very little of the race course in actually in the parks, rather on access roads on WDW property. Not a criticism ... just an observation. The CM do a great job of fan support. When I ran the parks were closed (race starts 6am) so the only spectators in the Parks were CM's.