Try interval running. Start by warming up with a brisk walk for one minute. Then run as hard as you can for a full minute. Make sure you run the full minute. Then walk briskly again for a minute, then run again. Keep doing this for three miles.By alternating the pace, you alternate the load on your heart and lungs and give them more exercising than running at a steady rate. This also changes the way you leg muscles work.You need to run longer than the goal distance to build up your endurance. I am not sure you can shave a couple minutes off in a week, but this is the approach that worked for me.