I'm with Machka.

1 month isn't long at all and your body hasn't had time to adapt. Keep riding, I'd say within 6 months you'll have your speed up a bit.

And 1-2mph is quite a bit of an increase when you're sustaining it for an entire hour. If you're on a hybrid and not planning on doing any serious racing, don't stress too much over your mileage and how fast you're going. Riding is a lot more fun without racing against your Strava.