okay
then you will be fine for the cutoff
(13.1 miles in 2.5 hours)
Because of this cutoff, o wouldnt start walk run till half way
that isnt the way i would normally advise to do run walk-the idea as has been said is to walk early on, and not just when you cant run any more-therefore conserving energy and being able to keep going for longer
you are going to possibly have a really bad time anywhere between 16-22 miles
if you havent gone really long befoe-you wont know what happens
so prepare to feel bad at some point, and want to give up, i mean REALLY give up
but, you get through it, and you finish-will be one amazing feeling
I think you have time for one 16 miler next week-and if you can do that, then its single figures to go afterwards
good luck