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.
It's important to know what MP feels like because you want to nail that pace in the early miles, which is hard because by the time you run your marathon MP will feel really easy at first, but of course that's because you're going to stick at that pace (or close to it) for 26.2 miles.
Setting MP based on a target time is a little bit dangerous, because when you toe the line there'll be a pace that you can sustain for the distance. Problem is, that could be 3:45 pace, or 3:50, or 3:40: you can make a pretty good guess based on your training, but ultimately you'll only find out later on the course. If you guess right, you'll have a perfect day, if you guess low you'll run strong, enjoy your day but ultimately feel that you could have gone faster, if you guess high you'll start to struggle but may not fade too much, but really overstretch and you'll crash and burn.
Look at the results of any big marathon and there's always a whole bunch of people who run the first half just under 1:30 then fade badly. Those are the people for whom the carrot of 3 hours was too big: they could have run a solid 3:05 or 3:10 but gambled and paid the price. Same at 4 hours. You're in the fortunate position that sub 4 would be well within you, and based on your training paces 3:45 probably is too. Nearer the time though, you'll have to be realistic over what you think you're capable of: you'll have an idea of what you might be able to do if everything went perfectly and you'll know what you could do comfortably. Where you set yourself is a matter of balancing risk against reward: how badly do you want that ideal time?
