The night before the marathon, pack a small bag that you can check at the start and retrieve at the finish. Some marathons provide bags that you are required to use if you want to check them at the start. In that case, be sure to use that bag or you will not be allowed to check your gear. Make sure the bag has a tag with your race number on it. Many marathons provide a matching baggage tag that accompanies every runner's race number. You can detach this smaller tag and put it on the back that you'll check at the start.
What should you put in this bag? Eventually your warm-up/cool-down clothes will go in it. But also pack your alternative race clothing options, and a Ziploc bag with the following: