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Thank you all very much for your help.
As hippy says, I get round in relative comfort but just get slower and slower as the ride goes on (who'd a thunk it?). I suppose that one of my worries is that one of the ways that I deal with getting through an audax is to try and keep it to a reasonable length of time, and that was possible with a 300, but probably isn't with the 400.
To that end I am actually more concerned about the 400 than the 600. My 300 took me 13 hours all told, which means that I am thinking in my head that the 600 is basically two 300s separated with 6 hours sleep, which I think is probably plenty (or does it not work like that in practice)
Anyway, thank you all for your help.
Fair enough, I missed that bit in the twelfty posts I was replying to (on the general "how do I train for Audax" theme).