A week ago I went to a dinner together with my girlfriend and some of her friends. The subject of running came up, like it mostly does when I am present. The guy we visited is planning to run the same half marathon as me in May, but have never really tried long distance running before. He has already talked about this with some of his friends, and he got the advice to prioritize interval training. This is the wrong advice!
Intervals - Trains your heart to pump more effectively. Very general and not sport specific.
Speed training intervals - Trains both the heart and running efficiency, and is believed to activate more muscle fibers.
Endurance - Repetitions makes your body move more efficient. Increases the body's capacity to store carbs. Very sport specific.
Intervals: I'm not saying that training intervals is a bad thing. Intervals are used for one thing, and only one thing alone; to improve the amount of blood your heart can pump through your body. It's essentially a way to train your heart. The result of the increased heart capacity is that your body can work at a higher rate than before. Intervals is by far the best way to get a stronger heart, but not the best way to become a better runner as it is general and not sport specific. This means that you can just as well do these sessions on a bike, on cross country skis or rowing with the same benefit.
Endurance: Just like with everything else that require technique, practice makes perfect. Repetition is the key, and the amount of repetition you get by doing intervals are not even close to the same amount of repetitions you'll get from running slow and long. It's like when you started coloring when you were a little kid. Keeping inside the lines were very difficult, but after a lot of training the same task seems menial. Running is the same way. The more you run, the more efficiently your body can make the repetitive movements, decreasing the amount of energy you use. But keep in mind, running long distance doesn't make you a better swimmers since it's very sport specific.
Another effect you get from longer runs is the increased amount of carbs your body can store. A long run over 1.5 hours forces the body to burn fat. The body compensates when burning fat by enlarging the storage capacity of carbs for next time. This affects how your body performs under all types of stress.
Speed training intervals: The middle ground here is the longer intervals that in Norway are called fartslek (speed games). This requires you to run longer intervals of 8-12 minutes with active breaks of five minutes between. A interval session like this can last well over an hour with a very high intensity. It doesn't train your heart as well as a fast interval session and doesn't train your running efficiency as well as an endurance run. The added benefit to this activity is believed to be activation of dormant/waiting muscle fibers that can make it more effortless to run.
Both intervals described in this article are hard workouts. Even professional athletes are very careful when doing these at a higher rate. 2-3 hard workouts per week is more than enough, as more will result in over training.
Summary: Intervals are a great tool when improving your overall capacity, but the most important thing when becoming a better runner is to improve your bodies movement efficiency. Becoming a better runner is not a sprint, but a long process of fine tuning your body.
Source: Tri - Torbjørn Sindballe