Yes, you've heard of 'em, maybe even giggled a bit at the name, but Fartleks can be a staple in the running diet.
The great thing about a Fartlek is that you can apply it to almost any run, and you can adjust it to fit how you feel on any given day.
Fartlek is swedish for Speedplay.
Take a run of any distance and keep the warm up, keep the cool down but sprinkly some Fartleks in the middle to mix up the pace, loosen up the legs, and open up some speed. In the middel look ahead and pick a mark - it might be a red car, a tree, a bison, whatever - and tell youself "okay at that car I'm going to pick up my pace 10%, or 20% or more and I'm going to hold 'til the corner". You can build your own intervals, make 'em last 2 minutes or 6 minutes. Make 'em ultra hard or pretty easy. What ever you decide make it a noticeable difference from your "go all day pace".