Carl, when you say you're doing some MP sessions what are you basing that MP on, 4 hours or 3:45? There's quite a difference between the two, over 30s per mile.
Dave, excellent question and post. This is so important and I must admit that when I was talking about doing some MP runs in February I had no idea about all the different things I need to think of.
I get it that it is so important to get the feel for what running at MP is al about. at this moment I think that MP for 4 hours is well within my capability. That said I have not got under 4 hours in my last 2 attempts and that will always be nagging doubt in my head.
But can I run consistently at 8:30 mm.
My tempo and interval sessions would suggest that this is possible but as you say it will very much depend on how I feel on the day.
My current thinking is to do my MP training runs at 3:45 pace initially to see how it feels. If I can sustain it then I will keep working on this. If not then I will have to readjust.
While I am targetting 3:45 I absolutely must get in under 4 hours so i will not want to jeopordise this by running the first half too fast and then blowing up in the second.
After all there will be other marathons.
But thanks for getting this issue out there as it is so important to think about this and not wander into this section of training without understanding what getting it wrong might mean. 
Edited: 18/01/2013 at 18:53