I'm really talking about a tempo run in the distance running sense. A hard, but not all out, effort. You can do this for time or distance. I suppose if you think about the fastest you can cover a distance you should aim for about 80% of that. You should be working hard and but able to stay relaxed(ish) and concentrate on good running form. I wouldn't go for more than 30 minutes.
You could always replace this with longer repeats say from a mile upwards.
The concertina effect is a bitch, but easily remedied by getting to the front and staying there!
I'm sure you will be able to become a strong runner by the time you get there.