Few days left

Sunday, February 17th I will take part in Tokyo Marathon. I was really looking forward to run this event last year as I had brought my mileage up to 400km a month in order to have an outstanding marathon debut. However after some health problems at the end of the year, this excitement turned into fear. And this, now feared date, is just a few days ahead from today.

The original plan was to run 400 km per month during the months of October - January and and begin tapering in February. Included in this 400 km would be a half marathon and a 10km personal best. Unfortunately, neither the half marathon nor 10km races took place, nor was I able to run close to 400 km. I did do it in the month of October, but November and December both averaged about 100 km each.
January, in a desperate attempt to catch up, I ran 350 km and have kept the long runs until last Sunday, just a week before my first marathon.
Another important part of the plan that was left aside were the 30km and over long runs. I was able to do only one (in October) instead of the two per month planned.

With just 4 days left until my debut I will begin tapering.
I will be happy if I can finish and still enjoy it. Very happy if I can finish under 3 hours. Finishing under 2.50 though (original plan), is now out of the question.

I wonder how it will feel?
I know I can run 21 km in 90 minutes and feel perfectly comfortable. Will that sense of comfort continue for another 90 minutes at that same pace?
I know I am in condition to run about 26 km in about 1 hour 36 minutes, but I would feel very very tired at this point.
The idea is to run at about 4 minutes 13 seconds per km pace for the entire 42 km. This should be easy to do without even looking at my watch for the first 20 km. I'll probably even feel cold with such a pace, but what will happen after the 25km mark? the 30 km mark? Will I even get to the 35 km mark?

At least the weather forecast says it will be cool ...