Wait, are you talking about 30 minutes "pedaling time" or "trip time"? My bike computer only counts pedaling time (it stops counting when I'm fully stopped at a light). A 30 minute "trip time" would be very, very hard to achieve in city traffic, but I think it's within the bounds of the laws of physics. I think a 30 minute "pedaling time" is more reasonable - within the physiological limits of most people, although it represents excellent fitness.

My best "trip time" is 1:07 for my 20 mile round trip commute, which I was doing 3 days a week. Minus the lights, that registered at 20.2mph average on my computer. My "cruising speed" on flat ground with no wind was around 21-22mph back then. I was no athlete - amateur racers should be able to cruise at 22-25mph for an hour on a road bike. I ride a mid-low-end road bike with panniers.

(Right now I only commute 1 day a week due to the baby and the toddler, and my average is now 17-18. No routine = no endurance.)

Get a heart-rate monitor and learn to do intervals. When you do intervals one day, back off the next day for recovery. A chest-strap hear-rate monitor is inexpensive ($40 for mine), and it is worth owning, even if you don't intend to race, although the physiological feedback is not as precise as a power meter (or so I've been told).