Any of the races are great for first-timers.
Your options are...
WDW morning races
WDW Half Marathon (January, Donald Theme, MK/Epcot course)
Princess Half Marathon (February, Princess theme, MK/Epcot course)
WDW night races
The Tower of Terror 10-miler --> obviously not a half but may be better for a beginner? (October, Villains/Tower of Terror theme, WWoS/Hollywood Studios course)
The Wine & Dine Half Marathon (November, food & wine theme, Animal Kingdom/Hollywood Studios/Epcot course)
Disneyland races
Tinkerbell Half Marathon (May, Tinkerbell theme, Disneyland/Anaheim course)
Disneyland Half Marathon (September, General Disneyland theme, Disneyland/Anaheim course)
So I guess you'll want to decide, does WDW or Disneyland appeal more? Morning or night race? Are characters on the course important (morning races tend to have more entertainment)? Do any go the themes of the races appeal to you more? Which course sounds like the best? What time of year can you actually do the race/train for it?
For a first-timer I think I'd recommend a morning race...personally I think W&D is my favorite half at WDW. But the morning races have more characters and the run down Main Street/through the castle, which it hi no is what most people think of when they think of running Disney.
Also...a 30-minute 5K is pretty good, in my opinion.