If most of your runs are carried out in the comfortable, aerobic zone, chances are you are holding yourself back from becoming faster.
Your body responds to the stimulus of training by adapting and becoming stronger. The essential ingredient in this occurring however is what is known as "overload". This is the principle of asking that little bit extra from your body, causing it to adapt in the following recovery period (usually 24 to 48 hours).
Without overload, your performance will reach a plateau where you just don't seem to get any quicker. There are many reasons for hitting a plateau but one of the most common is simply not pushing yourself hard enough.
Long, slow runs really are the bread and butter of any distance runner's training plan but this will only improve your body's ability to run long distances slowly.
In order to run those miles quicker, you need to mix in a bit of higher intensity training where your body is working "anaerobically", or without oxygen. Of course, you always require oxygen, which is why you need to breathe, but when you push your body over the so-called anaerobic (or lactic) threshold you start to call upon different energy systems to provide the work.
There are various ways of improving your speed but one of the simplest is what is known as fartlek training. Developed in Sweden in the 1930s, farlek literally means “speed play” and requires you to vary your running speed pretty much randomly during a run.
This differs from interval training in that is it not as prescriptive and you simply increase the pace as and when you feel like it but the principles are more or less the same.
Physiologically, your body improves its ability to work at these higher intensities which results in your “comfortable, easy pace” becoming gradually quicker.
Fartlek training should be gradually introduced to your training program, with no more than a session a week to start with. Simply take any one of your runs (except the weekly long run) and introduce 20 to 30 minutes of speed play into the middle of it.
Vary the pace by setting yourself landmarks to run to, slow down to the next landmark then pick it up again.
Your fast pace shouldn’t be all out, but you should be pushing yourself hard enough that holding a conversation would be difficult. When you reach the end of each piece of fast running you should be feeling the effects of lactic acid building up – heavy legs, heavy breathing, high heart rate etc.
If you are not already fartleking on a regular basis, try and add in a dose of it this week. If you have an important race lined up in the next fortnight, leave it until afterwards to start experimenting. Fartlek training is high intensity and can be a bit of a shock to the system so ensure you stretch down thoroughly afterwards and plan an easy run or rest day immediately after the session.