JN2 - you wouldn't get very far in a marathon running anaerobically. Marathon pace is an aerobic pace. There are plenty of good reasons to slow the runs down, but it more for saving something for midweek training, getting the body used to efficient fat utilisation, improving glycogen storage etc.
This is not to say that no Marathon Pace should be run on the LR's. Progressive pacing on some runs is useful. Have a look for example on the McMillan site:
http://www.mcmillanrunning.com/articlePages/article/2
Edited: 04/03/2013 at 00:22