Fartlek. A fun way to develop aerobic fitness and mix up your speed training a bit. I think it's a stressless way to get some speed on.* Fartlek is the Swedish word for speed play. In this type of training you run continuously in a natural setting, such as a park, the road, a forest, changing the pace at will. If you feel like sprinting from one tree to the next, that's what you do. If, after the sprint, you're in the mood for a long, controlled effort covering one mile, go for it. A fartlek session of 15 minutes and surges of at least 5,000 meter race pace will lead to marked gains in aerobic fitness. (current read: Training for young distance runners by Larry Greene and Russ Pate)
This type of training is great for a variety of races from 5k to marathon by simply adjusting the session time and length of each surge.
* a week ago on the track, I was changing from my running shoes into my flats and some guy asked me why the change. I said, "I'm getting my speed on." ha! Putting cool shoes on and going to a big high school stadium track does motivate me to speed up! I love it!