Hi Marcy!
Congratulations on becoming a runner! It is a great sport and you will love being a part of the runDisney family.
As for pace, you are correct. You don't have to run the whole time (or even at all) as long as you stay ahead of the 16 minute/mile pace that begins after the last person crosses the starting line. So, this means that the farther up you begin, the more extra time you will have. Even if you are placed in the last corral, position yourself at the front to give you an extra 3-5 minutes in front of the pace setters.
Corrals are normally assigned based on your estimated time of finish. If you don't know, you will be put in one of the last corrals. This is mainly done for safety purposes. The faster runners don't need to be swerving around us in the back to get by, and we really don't want to feel the pressure of getting out of their way, or even worse, trying to keep up with them!
If you haven't already, be sure to check out runDisney's official trainer,
Jeff Galloway. He promotes a run/walk/run method that I (and a high percentage of runDisney runners) use. You can also find more information on his website www.jeffgalloway.com.
I hope this helps! As spring approaches you will be so glad that you started this new adventure. Warmer weather is great motivation to get out the door!
Please let me know if you have any other questions.