Yes and no. Fat burning is an important part of distance running, but you can get many of the adaptations without running in a fasted state too often, whereas running fasted (/low glycogen) can severely hamper recovery and performance. I feel like the intense focus on fat burning is a bit of a fad (pun fully intended). You should also realise that boosting fat burning capabilities comes at the expense of glucose burning, which ultimately means you lose the ability to go into higher intensity zones. At the very least carb up before your speed workouts...
If you want to set yourself up for a successful marathon, I'd say being skilled in fueling yourself during the run is probably more important than optimizing fat burning.
From my limited marathon experience (two so far), the fuelling up doesn't really work 100%, even at large marathons with good supply stations, so you'll have to be prepared for fat burning (the little bags of carbs don't really suffice I found).
In a normal (say up to half-marathon) running plan, getting faster and stronger by pre-feeding carbs feels good, but for the long distance it's a lot different.