W&D is a half marathon, not a full. That makes a big difference in terms of impacts on parks. The 5k and 10k on Fri/Sat will be well done and clear before parks open. Might be slight delays with buses going to some places between 6:30 and 7AM, but that's it. The half marathon on Sun starts at MK parking lot, and finishes at Epcot. But even that starts at 5:30, with all runners on course by about 6-6:15, which means the last runners should be done in Epcot by about 10. So early AM things might be tougher for you to get to depending on the exact route and where they need to reroute buses to, but that should be the biggest noticable impact. Epcot might be busier that day, just from people finishing and then also the party that night so people going back over early.
