treadmill workouts for idiots!

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OK Im an exercise novice and have recently started to try and improve my fitness and loose the extra 28 lbs on my body!!! I have few questions about how to use a treadmill to burn fat.
I have a manual treamill and am actually unsure of how to put it on an incline as I no longer have the instructions!!
So I was wondering how long and at what intensity I need to run on the flat treadmill for a good workout?
Today I did 20 mins 2 mins walking at 4.5 mph and 2 mins running at 6.5 mph alternately - is this OK or am I doing it wrong??
Thanks in advance x
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