I'm not sure about the marathon, I've never been there for that. I can tell you that when my mom & I took my sister down there for her college program in January, there were no lines anywhere! It was really nice. I think it was the second week though. You may still have the Christmas/New Year's crowd there with you. If that's the case, it will be crowded. I went one year from Christmas & I couldn't move! I am actually going down the 7th of Jan., so I'll be there a couple of the same days as you.
I hope this information has helped a little bit. I'm sure some of these experts here know more about the marathon though.