Dead right about the marathon, it's my first one and I really don't want to be in a world of pain as I enter the unkown territory of the last 4 miles/glycogen wipe out. I will probably readjust my aim/pace from mile 15 on, depending on how I feel I am but I strongly suspect i'll be between 3.05-3.25.
I can relate to your sense of the unkown too. Last 10k, the fact that I was suddenly able to find a strong sprint finish told me that although I felt I was pushing hard as hell, perhaps not quite as hard as I thought. I'm learning to try and embrace the pain and not succumb to it. Ok. embrace is stupid word really but i'm at least trying not to let it dominate my thoughts as I get tired. That's hard but also a big key to good times I think.
1.23 sounds very do-able to me, given your MPW. As always, a lot will depend on the "on the day" factors but from a pure pace perspective, your time suggest that it should be achievable. You then start to get close to the sub 1.20 class and you'll be signing up for the next one before you know it!
Edited: 02/04/2013 at 15:37