To register you need to log into the annual pass section of the Disneyland or Walt Disney World website and click on special events for annual passholders. This race will most likely sell out quickly since Disneyland races usually do. Registration starts at 9am pacific (noon eastern). Registration for everyone opens March 25th.
Update: As of 9:35am the Avengers 5k preregistration spots are already full! That is in 35 minutes which means that regular registration will fill VERY quickly.
Who will be running the Avengers Half Marathon in November?