"Developing your aerobic threshold is the most important training adaptation to get faster at the marathon distance because it lowers the effort level required to run goal pace and teaches your body how to conserve fuel while running at marathon pace. The more work you can do to improve aerobic threshold and your ability to burn fat as a fuel source, the faster you can run the marathon."
"Once again, let me pause upon a memorable period of my life. Let me stand aside, to see the phantoms of those days go by me, accompanying the shadow of myself, in dim procession." - David Copperfield Chapter 43
This is one of my favorite lines from Dickens. On so many levels, these words ring true in my life.