The biggest impacts from Marathon weekend are traffic/closures, and having to share the park paths with runners still on course. Weds is Expo at WWoS only, Thurs is Expo, and early AM 5k at Epcot (done well before park opens). Fri is Expo and early AM 10k at Epcot (done before park opens). Sat is Expo and Half Marathon, runners may still be at the Epcot area when the park closes, but the biggest impact with be the World Drive down to one lane early AM in the MK/Contemporary area. This mostly affects people trying to make 7-7:30 AM ADRs, and any early morning MK tours. Sun is the Full Marathon. Same issues with early morning MK area road closures, and then runners will still potentially be in AK, DHS, and Epcot once the parks open. AK will clear out first, with DHS not clearing out til 11-12, and Epcot still having stragglers as late at 1:30-2.
All that said, yes, runners do go to the parks. However, many choose not to go to the parks as well, choosing instead to rest or rehab. (We need to be up & on buses at 3-4 in the AM, so have a full day already before parks even open) So while the hotels and resort restaurants may be full, the parks aren't that bad. Key thing is to allow for extra time when going anyplace in the morning Sat/Sun, and try to take Disney transportation whenever possible as the drivers should know the detours already.