I generally follow the following rule
- above 60-70F - a singlet (or tank)
40-60 - short sleeved top
below 40 a long sleeved top (in wicking material)
These are my general ranges but even my mileage varies depending on wind, and how I am feeling.
Pre-race
I will have a throw down cotton long sleeved shirt on if below 60. I usually pack it in the checked bag if the weather warrants but haev donated quite a few in the starting corrals.
If windy or below 45-50 I will have my
walmart cheap $6 sweatshirt that I donate in the starting corral (or pack in the checked bag if I feel like I do not need in the race).
In the 30's I will start the race in the long sleeved shirt and possibly the sweat shirt (ala 2010). I will make sure that I am warm and comfortable - not too hot then toss the shirt off course near an aid station.
You can just tie the 'throw down' clothing around your waist if you do not want to loose. Note that the mouse launders and donates thrown clothing.
FOR Bottoms... I have a pair of flannel sweats with the long seams cut up the legs to allow the removal of the sweats with the shoes still on.
I carry a trash bag with me for an emergency pancho and wind shell. While I started this in 99 I have only used once.
NOTE - look at the winds at Disney at the start. If the wind is out of the north do not toss your top too early. The course starts towards the north and bends to the west while in the protection of trees and hundreds of runners you may feel warm. Keep your top until you hit World Drive and see how the winds feel once in the open and running into the wind.
Long but hope it helps.