Discounted theme park tickets offered through runDisney are only available for a certain number of races each year, and it varies year to year which races might offer these specially priced tickets. In 2014, the Disneyland Half Marathon Weekend and the Disney Wine & Dine Half Marathon Weekend, were just a few that had this offer available. However, if you are participating in a race event that does not offer discounted tickets, you may still be able to take advantage of special pricing through GET Sports Travel, a runDisney event partner. I've used this company several times myself, including the Star Wars Half Marathon Weekend that I've just returned from. To get started, visit the GET Travel Sports website and under the Events Tab type runDisney. A list of races offering discounted tickets will be displayed.
I hope this helps, Julie! I would also love to know which race(s) you are interested in so I may provide even more detailed information on how to get specially priced tickets.