It's only a half marathon I believe.
16 miles is more than enough.
The last 3 miles will be tough for a newbie but most get through it.
However a base of a minimum of 6 months of endurance milage would really help. I guess many dong the GNR wouldn't want to commit to that but see it as a trophy/charity event.
I should have been clearer, the 16 miler was the long run when Marathon training, not half Marathon which is what puzzled me.