Jen -- based on what I remember from last year, here is how the bag thing works.
When you go to the expo, your goodie bag is a plastic drawstring bag. On race morning, there is a tag part of your bib, IIRC, that you attach to the bag. You tie the bag and then put that tag on it. We checked ours pretty much right when we arrived at the start area, long before we moved to the corrals.
I don't remember much of what I put in it last year, but this year I intend to put a dry shirt, some baby powder and some wet wipes (as a courtesy to anyone who has to be near me after I've done 26.2 miles), a bottle of water (I can't drink DASANI) and my
crocs. I am sure I will think of other things to put in it, but right now, those are the most important to me.
It was extremely well organized last year and it seemed many, many people used this service. There were a bunch of tents with separate entrances and marked with a sequence of race numbers -- 0000 to 100, etc. and you went into the tent where your number was posted and you turned in your bag and then walked through the tent to get to the start area.
After the race, you went to the tent with your race number and there were long tables set up with the bags laid out on them, and the volunteers handed you your bag -- by the time I got done, there weren't that many bags left.
