Excellent question on the "slow" pace for your long runs.
Your training program should have several components. You should be doing some speed work, tempo runs, fartleks, etc to improve your speed and train your body to run at a faster pace than you run for your long runs. The function of the long slow runs in a marathon training program are to train your body to burn its fuel efficiently and to get you used to running for something close to the amount of time you will need to run to complete your first marathon.
Remember that you will be running upwards of 40 or 50 miles total during the weeks of your longest runs. After months of training you will begin to experience what I refer to as Accumulated Mileage Syndrome or AMS. (Having a name for it like this gives you the belief that it can be cured) In other words, you'll be tired at times either mentally, physically or both. Before you run the marathon, you will taper for about 3 weeks which means you'll arrive at the starting line with pretty fresh legs relative to the ones you used during your longest runs. By running a certain portion of your mileage at a faster pace than your MP you will be prepared to make it happen on race day.
Everything that goes into the actual race day execution of this is another story altogether. You will get to experience that in the Fall. Good Luck. The journey to the starting line is the hard part. The race is pure gravy.
Post a reply on the Bulletin Board