Early Wednesday Run: I can't possibly be that fast

Early Wednesday morning, I headed out for a 4-mile run around Lake Hiawatha.  And I had a blast.  Even better than Monday's run. 

Heart rate:  The big story of the morning was my heart rate.  It was crazy low.  Am I really running?  Why isn't it faster?  I guess that cooler temperatures really do have an impact.  I'm so used to barely making a 13:00 mile in the afternoon when it's 85*.  This run was so much better.  And Lake Hiawatha was gorgeous.