I agree with the just PP. If the course was certified, most likely it's that you weren't running it as tightly as you could. The course is not measured like a person would normally run; it's based on shortest distances/tangents. More detailed description at - hamptonhalf.com/hamptonhalf-GPS.html
And congrats on the first 5K! I did my first in 33 minutes, and now my 5K PR (2 years later) is 27:39, so I have no doubt that if you keep training you will break 30 in time!