I train with a club of strong runners and we do intervals twice per week. In the summer we run on grass or trails and in the winter, bike paths (where I live we have 6 months of winter, unfortunately.)
We have all kinds of interval workouts. Generally, the shorter they are, the faster we run. We measure by time, not distance, although we usually get from 10-12 miles each workout including w.u./c.d.
One of the hardest ones we did was 30x 1 min. with 1 min. slow jog in between. I know it sounds easy but after about 12 of those your legs are freaking tired (we use very hilly routes). This week we did 3 min x 3 (with 2 min jog in between) followed by six 30 sec. hill sprints, then 3 min. x3 back. It was absolutley brutal. Our coach tells us we are usually doing them too fast but when you get a group of competetive people out there, it's hard not to. Last weekend we did 5 min/10 min/15 min/10 min/5 min; it was tough.
Our coach is an elite ultra runner who gets a lot of his ideas from that Italian coach Canova (they met at a marathon in Europe and are in regular contact ever since). The workouts are super tough but they work!!!