You said you walk 10 mile no problem and can run 4mile.
Just run the first 2mile then walk the rest, you will be surprised how many peaple are walking after a mile or two (baffels me to be honest). And when you get to the 12th mile......Run then if you can.
Just enjoy the day really.......Its great!!!!!!!!!!
I have done the GNR 3 times now. Normaly im in the 'white zone' but found myself in the 'green zone' this year..... It seemed busy all the time, no space, always seemed some one was within 2 steps of me.