If the race is held in 2014, then May 3 would be the date based on both historical race dates as well as the voluntEar page. Though, I would not make non-reversable travel plans until after the DL half. The program for that race should have some mention of the race date.
I hear that they are looking for a way to spice up the weekend. May is a tough month for road racing in central FLA. Most racing in the area has pulled back to the 5 k length with a handful of 10k's I would doubt that the race would go out to a half marathon length based on that alone. My best guess would be a combination 5 and 10k weekend. Obviously these are my impressions and runDisney has fooled me more than once.
As currently configured, this is not a race that I would have travelled to WDW to run. Too much $$ and too far for a 5k. Now that I am a resident and I am running all races at WDW in 2014, I will run it - though I would like to see something to spice up the weekend. I would love to see a 5 and 10k combo and as a closer to senior runner than young runner - morning runs. Much cooler to run in the morning and less likely to have weather influences.