Interval Junkie --Nobby
This might give you a good hint at what you might achieve: http://www.mcmillanrunning.com/index.php/site/calculator
Looks like about 4 and a half hours might be a good goal for your first one.
2015 Goals: Chicago Marathon PR
Current Status 04/23: Having a good time running with grumps
Yes I think Im going to start at that pace, it I feel VERY confident at 20 I might up it.