Hey Ben, coming to this thread late.. How is your hamstring? I'm doing this event too and it's also my first marathon. A few things:
Check out the RW race time predictor. As the webpage outlines, this supposes that you have trained for the distance. http://www.runnersworld.co.uk/general/rws-race-time-predictor/1681.html
If you're following the basic 16-week plan your shorter runs should be increasing in distance each week (I'm doing the RW Garmin ready sub 4:30 and my Thurs run was 8 miles...) Are you following it?
Focus on those long runs - slow and steady - and then when you get home it's rest and recovery, focusing on the next run.
Just wondered how you reached 3:30 as a target and how attached to it you are?
Also, have you tried parkrun's 5k timed runs? You'll meet lots of seasoned runners who can provide info and direction - not directions
Bushey might be your closest event.
Good luck and see you in Manc!