What is the Key to Running Long and Slow?

Nice day yesterday so I decided to make it my long day.  10 miles went by very easily at just under 9 minutes per mile.  Which brings me to my "whine-of-the-day".  Not really a whine, just an observation.  Running slow is more difficult than running fast.

I began yesterday's run with every intention of doing a slow 10 miles.  Slow as in 10 minutes per mile.  NOT slow as in 12:45 a mile, which Sara and I ran in the Chicago R/R Half Marathon but slower than what I usually run.  I judge usual in the 8:30 range for a casual run.  The weather was crisp but not cold with little wind - really a perfect day to run.  Additionally, I decided to run without the usual audio clues regarding pace that RunKeeper  provides so I really didn't have any idea whether I was running fast or slow.  The goal was comfort.  And 10 miles at 8:56 later, I was done.  Comfortable enough to believe that another 3 or 4 miles would have been easy.  I wasn't winded and could have easily carried on a conversation during the whole run.  In fact, had you been driving  along Metcalf Ave., you could have heard me belting out "Born To Run" at the top of my lungs.  It was that easy a run.

So tell me - why is it so hard to run slowly?  Why can't I run slower when I want, at an easy pace?  I'm not very good and am not trying to run fast.  That being said, I can run 7:30 or 7:00 per mile if and when I want (not too long).  I just want to run comfortably.  For a long time.  Like 10 or 15 miles.  Slow.  What is the key to running long and slow?