The earlier comment that an 8 mpm pace is fast and perhaps unsustainable is correct. But let's assume that you can in fact run a marathon at that pace.
You will have several objectives in preparing for your marathon. These include things such as, but not limited to, speed, endurance, and recovery. Some days you will be tired and/or sore; you might ease back on your pace and distance for these days, making these "recovery runs". Other days you will feel fine and want to go as fast as possible, or do interval training at a fast pace. You will also want to do some long runs at less than marathon pace simply to build up your endurance. (There is a good article on the long run in the "Training Corner" of this site.)
So try various paces and distances. You will enjoy the variety as well as benefit from the various speed and endurance aspects of your training.
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