You asked for it* and this week in the Training Sessions is fartlek.
I'm sure most of you know what it is but people do it differently so I'll spell it out.
Fartlek is an unstructured speed session. Warm up then pick a point ahead of you, say the third lamppost, a tree, the end of the field etc. It can be any distance.
Then run to that point at a random pace; sprint, run, walk, or jog. When on that section pick the next point ahead and the pace and repeat.
Remember to keep relaxed, don't abruptly change speed but do it in a nice controlled way. Speed doesn't come from being tense and rigid.
* really you didn't, it's Matthew Smith's fault for mentioning them a few weeks back