I am in no position to give you advice, having never trained for a marathon myself, but here are a couple threads with what seems like solid advice. Definitely something I plan on trying out when I move up to the marathon. You may or may not have seen them already, but they seem like solid workouts to me.
http://www.letsrun.com/forum/flat_read.php?thread=2487659
http://www.letsrun.com/forum/flat_read.php?board=1&id=1718448&thread=1710554