Setting Goals

Since I have never trained from a book or anything before now I haven't expected anything really. When I got 6th place in school cross country one year, and 3rd the next, I was lazily training for 2-3 months before the event. A good week for me would be to complete three 3km runs in whatever times. Now I'm going to follow a thing called "periodization." You have different phases you go through until your goal event is on. My goal is always going to be the school cross country. The next one is 4km. I'm using this book called "The Complete Practical Encyclopedia of Running" by Elizabeth Hufton, and I'm doing what it tells me to do if I was training for 5km not 4km. But that's fine really, its not going to make any difference to the times I get. But I don't know what a realistic time goal is to make so I'm at a loss here. If I set a goal according to the book I should get between 25min and 35min. In training though, I am running 4.2km in 20min. So, (if you haven't worked out) I am running a slightly faster than 5min per km pace. But then would I still keep that up for another rough km? I dare say I could, so I would run faster on the day wouldn't I. Not to mention I've done only 2 weeks training. So I think I should aim for an 18min 4k.