Hi, my advice is to listen to your body, and do not worry about adjusting your marathon training program to get more rest. The first time I tried to train for a marathon, I adopted a standard beginner's program, and it was too much mileage for me - ended up with a minor stress fracture and had to drop out of the program. The next time, I designed my own program with much lower mileage overall, and incorporating a lot of pool running. It worked out perfectly for me, and I finished the race with pretty fresh legs.

Keep in mind that the more tired your legs are, the worse your running mechanics are going to be, and the more prone you are to injury.

So, my recommendation would be to insert a very light week - even zero running if that's what your legs are telling you - and don't worry about falling back in your program. If you've already done an 18-miler, I predict you'll have no problem finishing a marathon. You'll be amazed at the adrenaline you have in your first one. I had so much adrenaline that I couldn't sleep at all the night before - and I still finished!

Good luck!