Nerves before races are nothing unusual. You'll probably go through lots of different emotions between now and race day. Nerves now will lead to excitement as you get on the plane or drive and make your way to Disney. You'll be even more excited at the expo with all its energy and the buzz of thousands of other runners around you. The nerves will pop up again the night before your big race (whichever distance that happens to be), and you most likely won't sleep well, so be sure to get a good night's sleep the previous night (Friday night for the marathoners, for example).
Race day is a roller coaster of emotions too. Excited and nervous on your way to the corral. Excited as you walk to the corrals, hear the announcers, and watch the fireworks of the corrals starting in front of you. Nervous as your corral is next to go. The three minutes before you get to go, you always find yourself wondering what you've gotten yourself into. And then the fireworks go off and you get swept away with the crowd and your nervousness disappears. You won't feel that again all day. Your body knows what to do once it starts moving. Your mind does too.
A new distance is ALWAYS scary - at least to me!
It's scary to everyone
What helped me in my first Half was to think of it in segments - like, get to the first out-and-back, and then you get to a new part of the course. Then get to the 6-mile mark and then you have to get back "home" somehow - might as well run!
I like mental tricks to keep the mileage in perspective versus getting overwhelmed by the distance ahead of me.
Disney is a really good marathon course for breaking into segments. Here's how I recommend looking at it:
Start to TTC
TTC to Magic Kingdom
Magic Kingdom
Exit Magic Kingdom to right turn towards Animal Kingdom
Quiet and calm until Animal Kingdom
Animal Kingdom
Exit Animal Kingdom to WWoS
WWoS
Exit WWoS to exit to World Drive
World Drive off-ramp
World Drive to DHS
DHS
Exit DHS to Boardwalk/BC/YC
Epcot
Choir
Finish line
Medals->pictures->snack box->check bag->beer->pictures (so have your Magic Band handy)
None of these segments are more than a few miles long, and as you cross each one off, you've got another one to look forward to.