some beginner plans only go up to an 18 miler. After 20 miles it is down to mental strength. Being your first marathon you will get a pb. It is more important to arrive at the start line in good shape than try to do too much in training and not get there at all.
Cal - most mornings I don't eat before I run, for a long run I have a banana as I head out, for a race I have porridge at home then a banana before running. I can eat most things while running slowly, if I am doing a hard session though it is safer with nothing!
Well, booted myself out of the door - very windy! Decided that as I was only doing a short run I would make at least one of the miles faster than normal and I did despite being nearly blown into a field