Bertrand Barenghien: What is interval training?

Assuming you are already able to run about 30 minutes comfortably, interval training is a great way to improve your base speed and your endurance.
Interval training involves running a little faster than your aerobic speed (speed you are comfortable running at for long distance, you can also use your heart rate monitor should be no more than 80% of your max heat rate). This training also involves repetitions.
Before starting, be sure to stretch and run 5-10 to warm up.
Run for example, run 1 minute fast, then 1 minute jog/walk to recover, repeat this 6 times.
You can have several variations, personally I run for 400m then jog 400m. To be efficient, you need to keep your fast run within 10%(time wise), so you keep your top speed pretty much the same through each serie.
Don't do it too often, it is tiring. Once a week or once every 2 weeks is enough. It worked great for me to improve my 5K time and to prepare for marathons.