Hi Carter - the real issue is that I have a place in London, but having been dogged by injuries, I've been umming and ahhing about it. I know it probably sounds odd, but I don't want to just do another marathon just for the sake of doing one as this will be number 4, so I want to do it if I am fairly sure I will break 4 hours. I had to defer London last year with a slipped disc and the back is still a bit of a problem, hence building up slowly. My PB (berlin 2009) is 3.54 and I've had my confidence knocked after a disaster in Paris 2 years ago where I was on for sub-3.45 and then got severe dehydration and crawled the last 7 miles.
So, that doesn't really answer your question, does it?!!
I guess marathon is A, but in order to get to A, I need to do a good B (ie. HM)!!