We will be there at the same time, but will be staying until the 13th. If you read up on the Marathon activities, you will see that the runners start off very early, and I would imagine the people who are going to see their friends and cheer them on will be int he parks, but I don't think it will be crazy. But in anticipation, we are doing our non-park resort day on that Saturday, and since there seems to be a big celebration for the marathoners Sunday night (Jan 8) at Disney Springs, we will go to one of the parks and avoid that.
We are anticipating the parks to open up a bit on that Tuesday (Jan 10) since a lot of people may be staying in WDW on that Monday after the marathon weekend - like a long weekend. But it will be great! No matter what! See you there, OP!!!