We hear all the time about turning your body into a "fat-burning machine". In theory your body has enough fat stores to fuel a run for a damn long time, certainly longer than any standard ultra-distance running event. The beauty of developing a strong and efficient aerobic system is that your body is able to utilise fat as its primary fuel source, which facilitates running for an extended period of time without flopping to the ground in a sugar-deprived state of bonk-ed-ness.
I've had a few questions since this post, asking how keeping your "easy days easy and hard days hard" (EDHD) ties in with Maximum Aerobic Function (MAF) training; specifically, are they the same thing and if not how does it fit together? Before we begin, go read this introduction to MAF training (and more detailed information is in this book).
Most runners run too hard too often. And when I say too hard, I'm more often than not also including your 'on' gear in that category - a lot of runners have a "comfortable pace" that is too fast for optimal training load and endurance development. Add to that your elusive fast gear, and you're running too fast and/or hard way more regularly than you should.