Racing a marathon is a pretty strange thing, if you stop and think about it. Take a pace that you can hardly run for a few miles some days, then string together 26.2 miles at that pace among all the other variables that need to fall in your favor. It leads to a lot of mental games during training. I've experienced all of these as a runner, and I see them surface regularly as a coach. The question is, how can you become confident that you are running the right pace in your workouts so that you don't put yourself in a position to crash on race day? Or, more simply, how can you stop having the, "There's NO WAY I can run this pace for 26 miles!" freak out?
Why Marathon Pace Can Be So Hard, then So Easy: The reason that marathon pace can feel impossible some days and downright "joggy" on race day is because of the overload-adaptation relationship. Training for a race involves a progressive or iterative overload approach (in terms of mileage, intensity, etc.). Because of this, you are running all of your workouts on partly (or very) fatigued legs. You're not simulating the first X miles of the marathon, you're working on the last X miles. Once you allow for complete adaptation and recovery with a proper taper, marathon pace becomes easy effort early in the race.
Focus on Finding Your Groove: The mental approach you take to pacing workouts will help you become more confident, and will help validate you've chosen the right target in the process. When running marathon pace, especially as the segments get longer, you want to find a groove that you know you can control. You could run faster, but you're able to keep yourself in range. I say this a lot to runners: The marathon is an effort race, so ultimately it comes down to finding the effort level that you have the most control over, and letting that be your MP.
Run Marathon Pace In Various Circumstances: To learn control and build confidence, you should run marathon pace in many different ways. You don't need to use a long tempo each week as your only MP work. Run short and long intervals at MP, finish long runs at MP, run fartlek workouts of varying distances/intervals at MP, and so on. Variation is always your friend when it comes to preparing for the race.
Review Past Success and Trust the Taper: During the taper, pacing worries hit a fever pitch. This is when you need to look back at what you have accomplished in the current cycle as well as pervious cycles so that you can see all the good work that you've completed. You'll see that you've put in a lot of miles at your marathon pace! Then, all that remains is letting your body recover and adapt so that you can run strong on race day.