Hi Candi!
Thank you for bringing your question to the Disney Parks Moms Panel!
You've picked a great week to visit Walt Disney World!
Crowd levels won't be heavily affected by
Walt Disney World Marathon weekend traffic. The Touring Plans crowd prediction calendar is calling for your first day, January 7, 2016, to be the most crowded. It is predicted to be an "8" with a high of "10". The rest of your week is predicted to be between "2" and "4". Those are very mild crowds. I've been there often during marathon weekend and the crowd levels have always been fairly low.
If you want to avoid runners at the parks, just don't go to Epcot early on Saturday or until late afternoon on Sunday. Other than that, your trip will be totally normal!
I hope this helps! Please let me know if you have any other questions.