Allen finishing a 10 mile LSD
There are three types of training, long slow distance (LSD), speed training, and hill training (see left-sidebar for links). In this page we'll look at the wisdom of doing long slow distance.
Long slow distance is running at a moderate pace in which you can carry on a conversation and in which you aren't panting for breath and don't get side stitches (cramps) or a raw throat. Running LSD means that you're running aerobically, that is, your body is getting sufficient oxygen. Running LSD means that you're using moderation in your training and not pushing your body to extreme stress levels. Most of your training should be LSD if you want to run injury free. It's during your LSD runs that your body develops endurance.
As you run, you put your body under stress, and body cells are destroyed. During the 48 hours after your runs, your body reacts to the stress by rebuilding the body cells, and the end result is that you're stronger than you were before. At least, its supposed to work that way. The problem is that if you apply more stress than your body can handle, your body can't fully recover, and residues of stress remain. Over time, those residues can build up until the stress reaches the point where your body breaks down and injury occurs.
Many runners are anxious to develop speed and long distances, and they push themselves to reach faster and longer goals. This works for a while, because their bodies can withstand the stress and not break down. But, if the high stress levels are continued, the time comes when their bodies can't handle the stress, and the runners either become injured, or they become so tired that they often lose motivation and stop running.
Of course, if all you run is LSD, you'll never become a fast racer. If racing is your interest, you'll want to do speed work and run hills after you have a good base of LSD.
While developing endurance through running long slow distance, you can pick up a poor running-form, such as unnecessarily short steps, slow leg movement, and sloppy form such as leaning forward and looking down. The running of "strides" can help you regain proper running-form. They are explained in the page on speed (see left-sidebar).