Garch - I'm hoping to knock 9 minutes off my time too (urgh, it looks a lot when you write it down like that) and like you I feel like I haven't done enough speedwork. I guess we just have to trust what training we HAVE done, and just go for it on the day. There is such a thing as a negative split, though I've never really understood why that's such a Holy Grail - I reckon it's better to aim for a steady pace.
Red_Rach - agree, careful with the parkrun. You might be better off doing a gentle saunter round this weekend, give it three weeks after the marathon then blast your way to a PB on the back of the race fitness. You don't want to risk anything at this stage.