Assuming you are referring specifically to runDisney races, each of the race weekends offer a range of different distances. I believe each race weekend has a 5K, 10K and Half Marathon with a Full Marathon offered only during WDW Marathon Weekend. Most of the race weekends are scheduled very similarly, so I think your schedule, whether you want to go to WDW or DL, and whether you are interested in a certain theme may be among the determining factors for what works best for you. I don't think any of the race weekends necessarily cater to "newbies" (they certainly aren't discouraged at any of them either), so any weekend that works for you should be fine.
Under 30 minutes for a 5k is pretty sparse information to make any type of judgment but, generally, I think people can finish any distance if they train for it properly. So determining the best distance for you to run is also a personal decision.
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