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Following a specific time puts pressure on you that you don't need, and it doesn't allow for variables. For example, you might have a slightly slower mile before you make that pit stop when nature calls, and you'll make up for it with a faster mile because you will feel better.
Try to run the marathon at a steady but comfortable pace, and pick it up when you feel good, and try to hold the pace when you start to get tired. Rather than focusing on the fact that your last mile was 20 seconds slower than the mile before that, tell yourself, “I'm going to pick out a person ahead of me and pass them within this next mile.” Don't sprint; instead focus on catching up to that person gradually, without exerting more than 80 percent of your maximum effort. Once you pass that person (which may take only 200 yards), pick someone else, and before you know it you will have picked up your pace slightly, but not so fast that you feel worn out.