We have been there for the last three marathons.
The only issues I noticed was our first year at CSR, they ran out of hot water from the marathoners arising EARLY, showering and leaving.
And that morning getting into Epcot could be a pain.
I was in MK during the marathon and literally had the place to myself.
Getting into Epcot after the marathon would have been a real pain if you drove in. By monorail or bus it is not a problem.
Just like any WDW vacation, the parks are busier on the weekends, and slower during the week.
From our experience this has been a great time to visit WDW! Almost evrything is walk on. The only exceptions we have found are Great Movie Ride and Tri-spinner in DAK.
I have even seen where CM's are joking at RnRC that they are going to go outside and pull in guests just to have someone riding!
It is not unusual to be able to ride KRR repeatedly w/o even having to get off and reboard.
Even Fantasmic is a walk in at the last minute.
And getting a good seat for a parade requires maybe an hour to no wait, depending on the parade and timing.
I am not sure about the half marathon, but the full marathon starts at 6 a.m. So you see many coming in around 9 - 11.
Special busses are provided for runners to and from the race start/finish. And Disney does a terrific joib on that. Busses start like 3 in the morning or another ridiculous time.

Disney gets the runners there EARLY. Something like an hour and a half before race starts at least.