I ran Boston in 1985. It was one of my best experiences. I ran a lousy race and had to walk a lot but even if this fate happens to you I know that you will be glad you put up with the hassle of getting to the starting line. It was hot that day so hope for cool weather. Is the course tough? If you are in great condition and run conservatively, then no. If you are not in very good shape or if you push the pace hard then it can be a very hard course. Hopefully your training included a lot of long not so steep inclines and declines. Running downhill a lot is important in Boston. You can be in great shape and in the middle of the race your legs may cramp up due to the downhill sections. Wear lots of old clothes that you can discard right before the start or at the first water station. You probably own old clothes that you are planning on throwing away anyway or have not worn in years. This is the perfect day to put them to use.