Dave offers some very good advice. And here's what I would add to it:
The prediction tables are based upon race day variables being favorable: temperature, humidity, lack of head wind, lack of hills, etc. Since this is your first marathon, try to achieve at least one of the following goals: 1 - finish the race; 2 - run sub-4 hours; 3 - run sub-3:50.
To increase your chances of all 3 achievements, plan to run at 4-hour pace through 20 miles, no matter how slow this my feel to you. At mile 20 assess your energy level. Now you're in a position to pick it up, maintain pace, or shuffle on in. Remember, finishing is your first goal. The things you learn in your first marathon can be leveraged in your second one. Peace.
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