In about three weeks out from a marathon, I use the workout below. It's a great indicator of whether or not I'm really ready for the pace I'm targeting.
Alternating 10x800s done in a total of five sets of two reps...first 800 done in interval pace and the second 800 done at marathon pace. Once the first set is over, go directly into the second set. There is zero "recovery" between the 800s. The purpose of the workout is to make MP feel quite comfortable. Surprisingly, it does assuming all of the training has been put in up until that point.