Do the seasoned audaxers on here think that doing a 400kms a week before doing a 600kms is mad?
Possibly. Character building, perhaps, risky definately. If the 400 goes well it will give you a mental boost for the 6, but if it goes badly it will make the 600 a bigger mental mountain to climb, so you need to make sure you are well prepared for the 400. Also assuming this will be your 1st 400 you won't know how long you need to recover.
Not that I am a seasoned audaxer, but my 2 week build up to my (only) 600 was:
Mon - Thursday - 50K commute
Fri - Sat, rest
Sun - 290K (200K audax plus ride there & back)
Mon - Fri - Didn't even think about riding
Sat - Sun - the 600
I think the 5 days off the bike before the event helped.
My tips -
keep eating, & drinking, Zero / nuun tablets work for me in the drink. You will be amazed at how much you can eat.
I've generally stopped using energy gels & drinks, food seems to work.
The only "energy" products I bother with is the stuff with added caffeine for when I feel sleepy.
Make sure you get used to riding in a bunch, given the opportunity it helps to be part of a group.
I found "The British" harder than the 600, Dai. Which was probably because I was under prepared mentally for the British.