This is not uncommon, but don't get discouraged. The few tips that I can lend are probably not news to you, but very important none-the-less.
1. Don't go out too fast. Start your race day pace a good minute or so behind your race pace and then after a couple/few miles slowly then gradually start to pick up the pace. In essence this strategy is called "negative splits". Almost every PR or marathon winner has employed this strategy. There is literature out there that correlates going out too fast with final miles being less than optimal.
2. Pinny is right. Make sure that you are taking in some sort of fuel every 45 minutes or so. This will replenish your glucose stores and help with fatigue. I find that I need to refuel every 40 minutes to prevent a low and maintain a more consistent energy level. I find the most important fuel recharge to be the one around mile 21ish. Although it is the last thing that I feel like taking it is the one that gets me to the finish.
3. Along with #2 is making sure that you take in enough fluids and electrolyte replacing fluids throughout the course. I tend to grab fluids at every stop until after mile 20 or so and then I don't stop (basically out of fear that I won't be able to/get myself to run again). And actually I walk the water stops. Again there is literature out there supporting this technique. It allows you to truly ingest the fluids (instead of spilling most of them on yourself) and gives you a mini break.
4. This is something that I do and it is a personal thing. I like to take a dose of 600mg/800mg of ibuprofen before the race starts and around mile 13. This helps even out the discomfort later in the race.
5. Last, but not least. You have done the work by training and now just go with it. You have acclimated your body to doing the mileage, now let it work for you.
These are some of the tips that I can lend. I hope they help. I also would like to share some thoughts with you. Things that I have realized by many years of long distance running.
1. The 18 miles that you are doing right now are more difficult than those you will do in the marathon. Again, Pinny is right. The adrenaline will help you through much of the run, but more importantly right now you are running many miles per week before your long run. Leading up to the marathon you will taper, allowing your body to rest and conserve energy for the big day.
2. I don't care if you run a 6 min/mile or a 20 min/mile for the marathon. It also doesn't matter how well trained you are. The last 3-6 miles of the race are going to hurt, especially if you are pushing yourself to do your best. At that point in the race you need to regain your focus and push on.
3. Remember it is only pain. It will go away at the finish line. The 18 miles that you just ran, hurt. But, after you were finished the pain (unless you have an injury) was gone.
4. Remember to have fun on race day. This is something that every runner has forgotten to do in their career. Smile (it makes the pain less) and wave to people. Get people to cheer you on, the spectators love to be involved they sometimes need some coaxing. Towards the end of a race I will flat out tell them "I need some help here" and they will cheer, which will help push me for the next little bit.
5. Encourage other runners. It always makes me feel better. They may be hurting now and need your help, but in return when you are hurting the favor will come back to you.
I think this post is long enough (sorry). If I think of anything else I will post. I am excited for you and your first marathon adventure.