In reply to QuarryKid:
I'm in a similar boat although a bit more off the mark (3:08 in Paris). I'm targeting Amsterdam in October for sub 3, the plan being that it would be easier to get the long runs in during the summer (I live in the mountains so winter is tough to put quality miles in).
I've done a sub 40 10km, but like you rarely run on roads - my half marathon split was 1.29 too but I get cramps in the calf and hamstrings in the last couple of kms of a marathon.
I think more long runs at pace are needed but I've had most of June off with an ankle injury (missing the Mont Blanc marathon as a result) so struggling to get the form back now.