Day 7 of marathon preparation

Today is the 7th day of working out or should I say moving my body and I can see a difference. The first day it was very difficult for me to even complete 2 mins of running and I could not repeat that. Today, I could run for 3 mins after every 4 to 5 mins of walking. The improvement is not huge but atleast its there.
Some calculations - Let's say we want to complete a marathon in 6 hours i.e. 26.2 miles in 6 hours -->4.3 miles an hour. So either we run the whole marathon at a speed of 4.3 miles/hr or run at a higher speed and then walk for some time.....uhhhh......this does not seem very easy considering that today the speed at which I ran was 4.3 m/h and I was out of breath after every 3 mins I ran.....prospects of completing the marathon don't seem very bright considering my present height and weight :(
NB is taking this very seriously and would be very disappointed if this doesn't happen. Let's hope for the best and keep working on it.