Once last fall when the weather was especially nice I did a 30 mile route on my commute to work. I'll never forget the time, 1:59:59.0 and I think I was riding the LHT.

For me, 15mph is decent, considering it is watch time, not bike computer, so the stop signs and street lights have an affect. I've done 50+ miles @ 17mph or so, but that was rural and the longest ride I've ever done up to that point.

All that aside, it's hard to say what a good time for you is. I suppose it's whatever you can manage and still stay healthy and happy while you do it.

As for your bonafide cyclist concern, I sure as hell wouldn't worry about that.