I was reading an e-mail recently from a friend who is running the Chicago Marathon, and after I has asked how her training was going she responded by saying she had just ran 18 miles with the last two being a struggle and was "getting scared." I am sure that on some level it was a tongue in cheek comment, but there was probably a bit of truth to it as well. At this point in your training, fear of the upcoming race is not uncommon. When I was training for my marathon there were several points of fear along the way. Specifically, I remember a 16 mile training run, which at the time was the furthest I had ever run. I finished, exhausted, and said to my wife, "that was 16 miles; on race day I do not know how I am going to make it another 10!" That feeling was a little overwhelming; I truly had no idea how I was going to finish.
I dealt with this fear in a variety of different ways, but in the end it came down to one thing, Faith. I grew up Catholic and was taught from the beginning that Faith is believing without seeing, and I think this directly applies to marathon training. I would see varying degrees of improvement in my long run each week, but each week I was exhausted by the finish. I was following the Hal Higdon training plan and it had me doing three 20 mile runs, with a peak of 50+ miles per week. The notes in the training said that that total weekly mileage would build my ability to make it the full 26.2 without actually running that far in training. At the time I was not sure I believed that, but I knew that I had to have faith that someone who had been there before knew what they were talking about, and he did! If fear is not that best word to describe your feeling then perhaps doubt is a better one.
At the tail end of the training (3 weeks to go until race day) it is common to have this doubt. Probably more so for first timers, but also for veterans who are seeking a PR, age group victory, or the all important BQT (Boston Qualifying Time). This is where you have to "trust in your training" (sorry had to quote Yoda), and know that you have put in the time necessary and that you are ready. I know this is not easy, but all the hard work and mileage will pay off.
By now, you have also probably completed your 20 mile training run. The completion of the 20 miler provided a huge confidence boost over and above the physical benefits. It helped prove that I would be able to make it and I would finish. As athletes we use this doubt/fear to motivate us to keep pushing, working hard, and keeping our eye on the end goal. Use this fear to help you along the way. I acknowledge that there is not much I can tell you at this point that will magically cure this feeling, that said, know that all of the long runs, tempo runs, all the stretching, watching what you eat, and all the time invested will be enough. You will be ready, and you will be a marathoner!