With regards to corrals I can only speak from last years experience.
I arrived at the start around 715-730am. Moved over the the green area. Found a spot on the ground next to a tree. Put my plastic trash bag on the ground. I was wearing a lot of layers of crappy sweats plus brought a blanket. You can fit a lot in the bags they give you.
Just chilled on the ground trying to rest, sip some gatorade have a little bite to it, use the biggest urinal in the world, etc. Didn't really get any of the goodies they offered since the lines get longer.
Around 915-930ish dropped off my stuff. Kept my crappy sweats, some pretzels and some gatorade. Went over to the corrals.
In green last year, 1000-1999 and 3000-3999 have seperate corrals that were fenced off and you needed to show your number to get in. Of course about 10-15 minutes before race time a bunch of cops with numbers in the 18xxx+ were trying to get up front. But thats a different story. This is all done in the park not on the bridge. The other corrals were supposed to line up by signs but it probably was just mass chaos.
I know that we moved up to the start before the women's start I think?? They moved the 1000-1999 corral first, then ours, then I don't know what they did with the rest. We got to the start between some buses on our right and the start in front. With a minute to go, you see the flying clothes everywhere. Don't be surprised to be hit with a sweatshirt in the head. Then the gun goes off and I was over the start in 21 seconds and over the finish line 3:06 later.
The starting area never felt crowded and you will notice that there are a LOT of international runners also.
For the start, I think the most important things are:
1. Pay attention to the weather. We had perfect marathon weather last year, but not perfect sit on your but for three hours weather. It was ~43F in the AM, so bring a lot of cheap sweats that your don't mind throwing out. Remember gloves for your hands. Target sells the great, cheap gloves for about $1/pair that you wear. I threw them out after 3 miles.
2. You bag can fit a lot of stuff. Bring a blanket, bring a couple of trash bag (big black ones). Bring a paper, bring a book, a snack
3. If you don't think its cheesy, write your name on your shirt. You will be running in front of 2+ million people who will cheer for you!
4. Enjoy it, there is nothing like running in Central Park in November, there is nothing like that first moment you get onto 1st ave in Manhattan or 4th ave in Brooklyn. The course is so quiet with running at the start then bam your in brooklyn with a lot of people. Note that in Brooklyn for the first couple of miles, most of the crowd is on the east side of the avenue for the green/blue course, since the womens orange course is still sparse with runners.
I wish I was running again this year, but alas no lottery entry for me. I've got Boston to look forward to in April. I can't see how Boston will be as good as New York was last year. We'll see.
But... since they start herding people to the start line around 9:40, why would anyone arrive more than one hour in advance?
