Above all else, your training should consist of a gradually increasing weekly volume of walking, the key requirement of which is recovery. I would recommend three sessions per week, consisting of two tempo walks, and one long-distance walk to train your endurance. The most sensible way to organise your week would be, for example:
- tempo walk
- tempo walk
- long walk
The two rest days should naturally follow the long walk, providing you with time greater opportunity to recover when you need it.
Your tempo walks should ideally be performed at at least 6.5 kilometres per hour, since that is the average required for the test. Over the course of your preparation, however, you should gradually lift these tempo walks to 7.0 and 7.5 kilometres per hour so you develop a reserve of speed to counteract possible fatigue. If you cannot manage 6.5 kilometres per hour comfortably, begin with a speed that you can maintain with comfortable mechanics, and programme to lift the pace gradually.
For the tempo walks, begin with a distance that is slightly challenging, programming for a gradual increase across the training period. The long walk should begin at roughly double that distance, with your aim being to complete it as quickly as possible, without necessarily reaching the speed of the tempo walks. You should programme to increase this distance gradually.
If you are a beginner, your distances should, perhaps counter-intuitively, be programmed to progress more slowly at the beginning than in the later stages. That is, your progress should not be linear, but rather geometric. This is because small increases in distance can be very taxing on the untrained body; you want to develop a solid base of general conditioning before pushing yourself excessively. Once you have developed this base, rather large increases can be adapted to quickly.
(Note that the opposite is true when you are highly trained.)
Other than that, record keep a record of your progress, and do not feel as though you are a slave to it. Your training programme should always remain flexible, subject to how you feel you are progressing and coping.