The Mona Fartlek is a workout conceived by Steve Moneghetti, 4 times Australian Olympic marathoner, and is a staple with Australian distance runners. It's a very appealing workout because of its simplicity and duration.
Basically it's 20 minutes of hard running and goes like this:
2x 90 seconds with 90 seconds recovery
4 x 60 seconds with 60 seconds recovery
4 x 30 seconds with 30 seconds recovery
4 x 15 seconds with 15 seconds recovery
The 90 seconds efforts are run at 5k-10k pace, however as the efforts get shorter the faster they should be run, so the last 15 seconds are flat out. The art of doing this session is not to take the recoveries too easy, so in effect this is almost a 20 minute tempo workout with hard efforts. And if done properly it serves as a great indicator of what you could do for 5k at that time.
Also it can be done anywhere, but try and avoid doing it on a hilly terrain as that would mess up your recoveries.
Be warned this is only a relatively short session but it's very intense. However after doing it for a few weeks it will become easier and the 5k and 10k times will drop. Enjoy!
(Remember to have a 20 minute jog followed by stretches and strides.)