I'm a big proponent of thinking about HOW you do a workout, not just what workout you do. Just as there are good workouts and bad workouts (or perhaps "not so good" workouts), there are also right and wrong ways to go about doing a workout.
As a high school coach, I see a lot of young runners going about doing workouts the wrong way. While I do my best to correct them, coaching can only go so far. Experience, understanding, and fitness also have a lot to do with your ability to run well in workouts.
Let's say the workout of the day is 6x1000m at anaerobic threshold with a 200m jog recovery—goal pace, 3:45 per km. Novice runners usually go about doing the workouts one of the following ways:
1) Start out way too fast
This involves heading out 10 to 20 seconds faster than their prescribed pace for the first repeat, then hit the goal pace for a repeat or two before succumbing to fatigue and struggling home. Runners who work out like this tend to race like this, too. Besides the obvious problem with this workout (we're not hitting the right splits), we're also essentially practicing slowing down. That's generally not a good idea. Workouts should teach you how to run faster, not how to slow down.
Workout splits: 3:35, 3:43, 3:47, 3:51, 3:52, 4:00
2) Never hit the right pace
When this happens, splits are all over the place. The runner can't connect his internal running effort with his external pacing as measured by the watch. An inability to hit the right workout times is usually borne out of a poor sense of internal pacing, poor concentration, or lack of a watch. When you're just out for a run, you can tune out and let your mind wander. But during an interval workout, you must be present in the moment, monitoring the sensations from your body, and the splits from your watch, to ensure you're running the right pace. You can't tune out and "just run" during most workouts. Veteran runners can hit a given pace within a few seconds per mile, even without a watch—this is an ability you need to cultivate if you want to improve.
Workout splits: 3:41, 3:55, 3:40, 3:45, 3:49, 3:42
3) Sandbag the beginning, hammer the end
Being a "workout hero" or a "Tuesday All-American" is one surefire way to irritate your teammates and coaches. While I encourage runners to bump up the pace a bit at the end of most workouts if they are feeling good, this is NOT the same thing as blasting the last repeat all-out. There are two reasons why this type of workout structure is a problem. First, you aren't hitting the right pace during the bulk of the workout. Second, hammering the last interval all-out dumps a huge amount of metabolic byproducts (lactate, acid, etc.) into your muscles, and the point of most workouts is to try to avoid that as much as possible. Hammering the last repeat also teaches a tense, aggressive running style. When closing out a workout, it is good to pick up the pace if you're having a good day, but most of the time, you should be running "within yourself," finishing the workout feeling like you could have gone faster if you wanted to. This teaches relaxation and efficiency, which are critical for jumping to the next level of fitness.
Workout splits: 3:52, 3:53, 3:51, 3:52, 3:50, 3:33
4) Pace the repeats poorly
Even if you're hitting the right splits for your individual repeats, you can still be structuring each individual repeats improperly. Unless explicitly stated, the right way to run workouts is at an even pace. It's not efficient to speed up and slow down in a race, so you shouldn't be practicing this in training either. This problem is also connected with a poor sense of internal pacing, and this takes practice to rectify.
Workout splits: 3:46 (90, 91, 45), 3:45 (88, 92, 45), 3:46 (89, 89, 47), 3:43 (87, 90, 46), 3:45 (90, 90, 45), 3:42 (90, 90, 42)
So, what's the right way to go about running an interval workout? I actually think it's okay in most workouts to start out a bit backed off from your goal pace, and ease your way into it over the course of a few repeats. This lets your body get up to speed so you are running as efficiently as possible for the main body of the workout, especially if you're working out in the morning or during a block of heavy training. However, if you're doing a workout without many repeats, like 3x800m, you can't really afford to have your first rep be slow. This is where understanding the purpose of the workout comes into play. Is our goal in the interval session to practice an "internal load" like anaerobic threshold? If so, easing your way into the pace is a good idea. Are we trying to practice running a specific pace so you can qualify for the state meet? Then you need to be on the pace from the start.
During the bulk of the interval session, you should be hitting your goal splits, both for each repeat and for the individual 200 or 400m splits within each repeat. Over the course of the workout, your times should edge a bit faster as you get more focused and tuned-in to the pace you're running. If you feel good, you can squeeze the pace down on the last few repeats, but in most cases you shouldn't be hammering them. It's better to run within yourself, practicing efficiency and relaxation while moving fast.
Workout splits: 3:47, 3:46, 3:45, 3:45, 3:45, 3:43
To help you visualize this, I've included the graph at the top of this post of my own splits from a 16x200m workout I did a while ago. You can see in the graph all aspects of what we discussed earlier. Although the workout was a fairly fast session, I was doing it early in the morning, so I took the first few repeats to work down to the target pace. The workout times were very consistent, and at the end, I was able to push down a bit under the goal pace without struggling.
You can contrast this with the graph below, from another workout I did a few months back. This is what your workout structure should look like for a challenging workout at race pace. There's no opportunity to ease into the pace because there are only a small number of repeats, and the goal is the "external load"—the numerical pace we are running. Because of the difficulty of the workout, I did not push down the pace at the end, and this was definitely the right decision.
Once you're able to structure your interval workouts properly, you'll get a much bigger benefit from them; this benefit will translate into better training and better racing, both in terms of overall time and in terms of structure.