I ran the marathon this past Jan so I can give you a little advice based on what I saw. The full marathon on Sunday is the only event that will affect you at all (and minimally). I might avoid DHS and EPCOT that morning. The other events that weekend are done before the parks even open (for the most part).
I was still running through DHS and EPCOT around noon and by then those parks were pretty crowded. CMs held people back from crossing the course until there was a gap in runners. There were also a lot of people standing along the route cheering. It was similar to if a parade had been going on to get an idea of what I'm talking about. This might make it more frustrating to tour the park if you're really in a hurry, but otherwise won't really affect anything.
If you are driving your own car, be prepared for some road closures. Bus transportation might be affected as well, but all I saw was this outside of DHS...
Anyway, the general crowds were not bad the week after the race. I did not do any parks that weekend so I can't tell you about Sat/Sun. However, keep in mind that the following weekend is the MLK holiday and we noticed crowds pick WAY up that Friday.
It's definitely a great time of year to go! I hope you have a blast!