Interval training is extremely important. Whether you are a true distance runner with an abundance of long muscle fibers, or a fast twitch runner, like myself, who did 400's and 800's as a rule in high school, interval training is going to have the effect of either making you faster, or translating your natural speed into longer distances. But what is interval training, and what do you aim for in an interval session?
My answer to the first question is sustaining a speed that is faster than your 5k pace for a few minutes at a time, or maybe at a distance of 800m to 1200m, depending on your speed and where you are at in your development as a runner. Many other coaches will have many other definitions of interval training, but this is the definition that I use when training myself or other athletes. It should be around the speed that you could sustain for a 3200m run, if you are thinking in terms of high school races. Currently for me, that is just under a 5:20 mile pace. Your rest interval can be jogging, walking, or standing, but it should be equal to or a little bit more than the time it took you to complete your interval.
My answer to the second question has several parts. First off, you are looking for consistency. Your last interval should be within a range of 2 or 3 seconds of your first, or you aren't doing something right. If your first one is faster and the rest get significantly slower, then you are not going to have the positive effects you are looking for. As a coach, I don't care how amazingly fast you can run the first interval. I also don't care if you can run the last interval 10 seconds faster than the previous ones. It is the middle intervals that are the most important. If you are inconsistent, you are not getting the workout you need.
Next, I often find that the first interval feels hard. I might miss my goal by a couple seconds on the slow side of things, even if I've had a good warm-up. My body is making further adjustments during that first interval, so it's almost like an additional warm-up. At the same time, I never feel like the first interval has totally spent my resources. I just need the first interval to shake off the rust or get my body going with the flow of the rest of the workout.
During my first rest period, I often times feel fully recovered early, and am tempted to start the second one too soon. I've learned to resist that urge and go with a specific timed rest interval. After all, I don't want to peter out and be running too slow later on because I didn't get enough rest. Part of interval work is letting your muscles get in a rhythm of speed, so they can learn how to keep that turnover going for longer distances. If you start your second interval too early and end up running 10 seconds too slow as a result, you will not achieve the desired effect of conditioning your muscles for sustaining faster paces.
On subsequent rest periods, my heart rate and breathing are somewhat settled, but I'm not feeling quite as fresh when my watch beeps to tell me to get moving. While you should never start an interval feeling sick and out of breath, you probably shouldn't feel as fresh as you did before the workout began, either. A little elevated heart rate and harder breathing is to be expected. By the last one, you might feel done before you start, but the last one tends to take care of itself, so go for it anyways.
During the third interval and latter intervals, I often get that sick feeling somewhere between my stomach and intestines by the last 200 meters. I expect that, and I'm actually shooting for that feeling. It's always challenging to push through that feeling, but any seasoned racer knows it is an absolutely necessary skill to develop. By the last interval, I will often hit that feeling right in the middle and have to push through regardless. When I do push through, crossing the line at or under pace, I know I'm going to see some great results.
In the latter intervals, my legs are a little spent after the first 400 meters or so. This is when I focus on turnover. I move my arms fast, and my legs follow. I expect to feel tingling, or deoxidation, in my legs. I rarely feel it in my arms, however. Let your upper body take over the latter stages of interval work, focusing on keeping your head up, eyes on the horizon, and arms moving ahead at a rapid rate, never crossing the imaginary line down the middle of your body.
The interval run that I did yesterday (link below) met all of the requirements listed above and was very beneficial to my development in this race cycle. Two additional things to keep in mind in order to have a successful interval workout are as follows: 1) have a bathroom nearby, and force yourself to use it after your warm-up jog (nothing messes up an interval workout like an attack of the poops), 2) choose a course that is relatively flat if you are working on speed and turnover, and make sure it is a course you can run uninterrupted (i.e. you don't need to wait on a street corner for cars to pass).