runDisney events often sell out really quickly, so it's best to be ready to register on the day registration opens. The events/competition threads the previous poster mentioned are great and very helpful in preparing for the registration (which is a race in itself!) We have annual passes, and runDisney often offers early registration for annual passholders, so that helps!
As for opening additional half marathon spots later, that very rarely happens . . . but it did last year! Well, it was more like a redistribution. The Goofy Challenge (half marathon and marathon registration combined) did not sell out. So at one point last year, runDisney converted some of the Goofy Challenge spots into individual half marathon and marathon registrations. I was already registered for the marathon, but the late conversion allowed my husband and teen boys to make a relatively late-in-the-game decision to register for the half marathon. You might also investigate the charity partners listed on the runDisney website - they require fundraising minimums, but often have spots available on their teams for races that are otherwise sold out.
As for cancellations, registrations are nonrefundable and nontransferable. Participants can defer for a year (not sure of the details.)