How do I improve my 1.5 mile time big time in 7 day?

Question

I used to run track back in high school and was fairly good running long distance. I'm 23 now and weigh 140 ( 5'7'').

I'm running a horrible 1.5 mile time of 14:50 seconds now. I've managed to get it down 1 min with in 2 days of hard workouts. I have 6 days to get it down to a better time. I've been running 1.5 mile, walking about 400 meters , sprinting 100m's then walking 200 m's about 4 times then running another 1.5 mile. I've done that the past 3 days. How can I improve fast?