The decoration start coming down during the first week of Jan. So if you arrive for your stay before the start of the marathon, you might be able to see some of them. If you arrive for the marathon and stay after it is over, every thing will be down by then.
The Candlelight Processional is usually over by Christmas Day. Osborne Lights usually are over by the end of December. I think the Lights of Winter at Epcot are about the last to be taken down. Every marathon I have been to, they were lit up in the morning for the run through Epcot.