Just to give you an idea here are 4 training schedules for four different finishing times
http://www.209events.com/file/250.pdf
Every one progresses differently, have a look at the attached and see how you get on in the race you have planned then set your new target based on the result.
Improvement will come easier at first but the faster you get the more work it takes to improve, eg. I knocked 2 mins off my 5 k time in 8 weeks but over the next 8weeks I only got a further 20 seconds off.
