There are at least six compelling reasons to add intervals to your workout, but don't go skimpy on the recovery portion. It's very important to the whole process. Coach Jenny, over at Runner's World, likens the recovery period of intervals to sleep. You need sleep to repair your body and rest up for the next day, and you need a chill-out period for your body after pushing it through an intense interval so you can push yourself through another one.
Jenny explains it like this:
. . . the payoff comes when you invest in a proper recovery and run at the target fast pace (and not any faster), your body will adapt and you'll soon be capable of not only running faster with less effort, but the time it takes to recover will decrease as well.
When beginning interval training, use the recovery period to allow your heart rate, circulation, and breathing time to recover so you can attack the upcoming intervals with the same energy. If this means you need to walk between sets, that is fine. But as you repeat your interval training, you will notice that you don't need to run as slowly to recover, nor do you need the same amount of time. Just remember, this change doesn't happen overnight — it's a process.