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Run Wales - what you say would seem to make sense, but you have to consider what your goal is - if you are looking to complete a marathon, then yes you will be fine with that approach. But if you're looking at IM it is important to work within Z2 . One of the key factors is the energy source your muscles will be using. If you keep in the aerobic zone, you use both glyogen and fat as energy - in varying amount. By training in this zone, you 'teach' your body to utilise more fat than it would normally - which is essential for IM (not so much for marathons as you're only working for 3 hours or so) as you will run out of glycogen rapidly.