I read through some post stating 20 is good and up to 28 or so considered even better......********** The average of a TDF winner taken over the last several years shows a 25 mph average. Now that is with a lot of mileage daily obviously but even then, if they wore out the flats for same distance I am sure that average would be higher. If you achieve things like this then you may want to consider a job change. I find it hard to think however as a beginner you are there to those numbers as I suspect like I stated, when the real hills set in, that average usually drops a bit.
But once gain, not what it is about. Having fun and getting out and exercising should be the main goal. Just have fun buddy.