The components to you running a faster middle-distance run are your VO2
Max, efficiency of your form, speed and strength of your muscle, body composition, and how your body deals with lactic acid. I'll explain these things, and then we'll move on to the training methodology to capitalize on copiousness them magical.
This is how much oxygen you can take in and actually use. Using more oxygen creates more ATP through oxidative phosphorylation, your aerobic metabolism. Aerobic metabolism doesn't really cause fatigue, so it's necessary to maximize energy output here. And to do that, all you need to do is increase the amount of oxygen you can use, which is your VO2
Max. Does this make sense?
Improving your VO2
Max has very little to do with breathing more though. Anyone can breathe all fierce, but hyperventilating and getting light headed isn't going to help your performance any. The amount of oxygen you can actually use is a result of several factors. It starts in the muscle. Because mitochondria within the muscle use the oxygen to create energy aerobically, mitochondrial density determines how much oxygen can be used at the muscular level. And this is the most important factor, but it's not the only mechanism for oxygen use. You still have to be able to transport it in a sufficient volume to the muscle. This is dependent on total cardiac output and the amount of hemoglobin to carry it through the blood. If you don't have enough hemoglobin, you don't transport enough oxygen, and if your heart isn't putting out as much total volume, the delivery is obviously hindered as well. And then there's a lung component as well, having the ability to transfer more from the lungs to the blood. All of these things need to improve though in order to increase the end product of oxygen utilization. And once you improve your max, you can also sustain it for a longer period of time- so it ends up doubly good, allowing you to run at a higher pace for a lot longer before fatigue sets in.
To get these improvements, you have to consistently challenge your aerobic metabolism to its full potential. In doing so, you'll get considerable VO2
Max improvement during the first 6 or so months of training, beyond what is achievable thereafter. The rest of your life, you're looking at improving something like 10% per year if you train in a way to target oxygen use directly. We'll get into the specifics on training in a minute, but for now, just remember that in order to maximize aerobic capacity, you really need to directly challenge your aerobic metabolism.
Efficiency of form:
The idea here is that you'll be training hard to increase your body's ability to generate the highest possible amount of ATP during the event. There's no point in improving the amount of ATP you generate just to blow it on extraneous movement via shitty form. It doesn't even matter what your distance is, the same rule applies to all of them: if your form isn't efficient, your performance will suffer. You should be using virtually all the ATP you generate to propel you forward. If you're not, you're bad at your sport regardless of your physiological athleticism. Talk to someone else about improving your form though. I'm not going to cover that.
Lose weight. The less fat you have the better, considering it improves your power to weight ratio, which is critical in your performance. You carrying around excess bodyfat just means you have extra pounds your legs are responsible for moving. You have to generate extra ATP to do this. It's a waste of energy, so lose fat.
Speed and strength of your muscle:
Different amounts and forms of contractile proteins in muscles cause discrepancies in strength and speed of contractions. This can be summarized with muscle fiber typing. Type I fibers are slower and weaker but have higher mitochondrial density- so they use more oxygen and consequently don't fatigue very much. Type II muscle fibers have a pretty wide range of strength and speed capabilities depending on contractile proteins, but on average, are significantly stronger and faster than type I fibers. They rely on glycolytic metabolism however and because of this, fatigue pretty fast. The optimal ratio of muscle fiber typing is that which can propel you at the pace you need, with just enough resistance to fatigue to last the duration of the race. Depending on your training methodology, you can change ratios of fiber types to attain the optimal level. We'll cover training methodology in a bit, but for now, just undersatnd that you want just enough type I fibers to keep you from fatiguing, and the rest type IIs to increase your pace.
Lactic acid is the largest cause of fatigue in these events. Pretty much any race lasting between 10 seconds and an hour, lactic acid is your primary source of fatigue. You need your leg muscle to work in order to get you through the race and when lactic acid levels get high, it begins inhibiting your muscle's ability to contract. Your faster type II fibers will be producing lactic acid during their use. At low intensities, you can shuttle it to the liver and kidneys and convert it back into carbohydrates for further energy production. Not here though. While you're using your legs all fierce to make you run faster, the blood vessels providing blood to the legs dilate and those providing blood to your liver and kidneys constrict. So the only effective way you can get rid of the lactic acid is to use it directly in the mitochondria of the type I fibers. This is why an ideal type I and type II muscle fiber composition is so important. You have to have the type II fibers to make you run fast, and you have to have enough type I fibers to use up the lactic acid produced by the type IIs.
Increasing the mitochondrial mass (mostly in type I fibers) improves the body's ability to metabolize more lactic acid and produce more aerobic energy, and as a result, offer higher ATP production without increasing fatigue. And the other thing you can improve is tolerance to high lactic acid levels. We'll talk about training methods now to get the greatest benefit out of these things as well as all the others.
How to train:
Early in the competitive season, everyone is out of shape. The whole metabolic apparatus by which you perform is out of sync with the demands that will be placed on it. The muscle isn't ready to perform, elasticity is lower, and the chance of injury is higher. Plus, all the benefits you could achieve from intense training at this point, you can achieve just as well by taking it easy. So at the onset of training, pushing yourself is the worst thing you can possibly do.
Do your first 6 workouts at a fairly easy intensity. Train no harder than the quitters do. Progressively increase the distance you run during each of these sessions so you can prepare the soft tissues as well as increase mitochondrial mass of your type I fibers.
The 7th workout, do a thorough warm up followed by a timed trial of the competitive distance at maximal effort. Then cool down, stretch out your hamstrings, glutes, quads, and hip flexors, and you're done. Remember your time. It gives you the pace to work from for the remainder of the practices, which are all pretty structured.
The rest of your practices are based on the results of your timed trial. Pick a goal time you want to reach. Make it challenging, but attainable. Do a little bit of math and figure out what pace you would have to run to achieve that time. Mark off a good number of interval distances throughout the length. After a thorough warm up, do timed runs at the exact pace you have to run at to meet your goal. Maintain pace as long as possible. When your time falls short of where it needs to be at one of the interval segments, stop running. Never continue to run anyway.
When you stop due to your pace falling short, walk at a slow to medium speed for 5 minutes. The walking during recovery helps metabolize more lactic acid, and changes your pH faster, preparing you to run again sooner. This entire rest period challenges pure aerobic metabolism. Because you're training above your full-length capacity, the running component is almost pure anaerobic metabolism. This would do very little for your aerobic metabolism without the rest periods. And to improve each of these components, you progressively cover more and more distance during each run, and you will progressively cut down on the rest periods to challenge the aerobic metabolism on quicker recovery demands.
Every day you practice, cut an additional 10 seconds off the rest time between running attempts. The first day of this program, you rest for 5 minutes between each run. The second day, 4 minutes and 50 seconds. The third day, 4 minutes and 40 seconds, and so on.
Additionally, the number of running trials each day varies as follows:
Day one: 3
Day two: 3
Day three: 2
Day four: 4
Day five: 4
Day six: 3
Day seven: 5
Day eight: 5
Day nine: 4
Day ten: 6
Day eleven: 6
Day twelve: 5
And so on...
You should hit your goal pace, regardless of what it is, before the rest period drops too low or the trial number climbs too high. As soon as you hit your goal pace, your practice is done for the day, and the next practice you start over with a new pace, 5 minutes of rest and 3 trials. Go through the exact same program. This periodization format will help combat injuries associated with overtraining while maximizing your results.
You should be training no less than 3 days per week and no more than 4 in order to develop the metabolic apparatus ideal for this length of event.
In this type of interval training, you perform at higher intensities, which causes the production of higher cumulative lactic acid buildup. This is what causes your adaptations to lactic acid buffering, metabolism (consequently threshold) and tolerance. And by causing adaptation at this level of intensity, you get a more ideal muscle fiber orientation as well.
The recovery intervals between running trials challenges your aerobic metabolism. This is where you get increases in mitochondrial density and VO2
Max. Watch the breathing of someone running a 400. It doesn't get all fierce right away. It takes a good 10 seconds before the runner begins to breath heavier, and following the run, they continue breathing like crazy for a good couple minutes despite their inactivity. There are a lot of reasons for this. You have to replace your oxygen stores in the blood and muscle, fuel your crazily-pumping heart, account for elevated metabolic processes, resynthesize energy substrates, and so on. This is called EPOC (elevated post-exercise oxygen consumption), and it's accomplished aerobically. That's why you breathe like crazy (remember aerobic metabolism is dependent on oxygen). So if you focus on recovery time improvement, you get increases in aerobic capacity.
All in all, this progressive interval format makes you a badass middle distance runner.