Lit, I've just looked at gfa and all other ages start at the round number ie 50-59 and 60-69. The 41 thing is a bit of an oddity.
I hear what you say re other marathons. However, if I'm essentially dedicating 20 weeks to this and have already experienced London, I'm worried something else that doesn't require a ballot may not seem as worth the effort. London was such an immense event it seemed a fitting conclusion to what id put in.