Congrats on reaching 30 miles. If it was your longest ride yet, don't worry about the average speed. I have never been able to maintain my "normal" average speed on a new distance record for me.
For speed, are you doing some sort of interval training? Take one day a week, and after warming up ride as fast as you can for 2-5 minutes, then ride at a slow pace to recover for 5 minutes or so, more if you need to, then repeat. You can also do this with hills. This should help increase your speed.