Hi Karen!
RunDisney races are very popular and they all eventually sell out! Some sell out within a matter of hours, while others will remain open for a few months.
There is not a wait-list option for the races, as of now. Most of the time, if the race has sold out, you can still find spots available through charity groups or travel operators. These groups secure a certain number of spots and fill them as requested.
You can find information about each race weekend's charity and travel groups on the
runDisney website. If you click on the event weekend you are looking for, then the tab "REGISTRATION", you will see tabs for "charity groups" and travel operators".
If you know what race weekend you are interested in, please let me know back on this panel and I will help you find some options for running that weekend!
I hope this helps. Let me know if you have any other questions.