After a couple months of "just running miles" I raced the SF half marathon this past weekend (I did the easier 2nd half unlike the brave souls in half #1 who obviously prefer hilly races that start at 5am....)
If you are into long winded race reports, you can find out how it went here as I attempted to get under 1:25 for the first time...
Alan, that is a a solid race and a hefty PR, especially since you have been "just running miles".
I have had the experience you describe once or twice, where you realize partway through the race that you are ahead of pace, but you're feeling good enough to keep it up, or maybe even give a little more. It is quite a thrill. Excellent racing.