We were at SSR last year during the marathon and it didn't bother our vacation at all. The morning of the both marathons taking Disney transport is a good idea, they know where to go to avoid any road problems. We got to MGM as runners were making their way through. It was pretty neat to see actually. Some people in speed whell chairs, so awe inspiring. There was charters cheering people on all along the path. It was all over by 1:00.
The parks didn't seem overly crowded and the rest of the week is rather empty.
This year we are staying at VWL (our home resort) we are actually wait listed for BCV, but I don't think it is going to come through, just because of that weekend.
We go during that week for the same reason (DH's teaches college and doesn't start back until Jan 13th. We will be there Jan 3th to the 10th.)
It can get cool in Jan, pack accordingly. We made our ADR's last week and got them all (but HDDR, they said we couldn't book it yet.)