With a marathon you will get hundreds of different of opinions. I have run 2 and not particularly fast but stick to 18 miles as my longest training run, I have spoken to guys who can run a lot quicker but only run up to 16 miles other will run past 20 miles. It is important to listen to your body. A lot also depends on conditions of the day, course and whether your head is in the right place. If you are worrying about injury seek advise before deciding whether to go to the start.