Long Run

I haven't started my formal training (or maybe I have) yet but I did manage to take advantage of the great weather this past Sunday (11/11/12) and fit in a long run.  I ran 16 miles in the Cleveland Metro Parks in 2:19.47.  It was a great pace for me and I felt good afterwards... tired... but a good tired.  Here's what I debate in my own mind and would like feedback from the masses on; what are the physical benefits of long runs while not part of training?  Not so much the benefits maybe but who does them?  How often?  Do you build up the miles and then back down only to build back up again during training?  In the back of my mind I think that these long runs outside of my formal training plan will help me do better come marathon day.  Thoughts?