Understand your food needs
... supermarket brain death' and the associated 'service station eating disorder', where one wanders the shelves in a trance like state searching for something you can bear to eat amongst the overpriced over processed pap on offer...
I'll say.
I find making sore I drink more than I think I need and having a 'menu' depending on time of day helps get around this. eg (though adjust depending on when you start etc.).
early in the ride/day: breakfasty stuff eg with bacon ies salt)
mid-morning to mid afternoon, some of: cheese and tomato sandwiches, yoghurt, raw carrot/apple/grapes, plain or seedy rolls (at supermarket)
or something like a toastie/cottage pie (ie not too greasy). I've even been known to eat beans on toast
mid afternoon: toasted teacake/scones/cake, milky drink
early evening/late afternoon: soup and roll
later evening: usually at a manned control so whatver on offer, else see breakfast or pizza-y stuff
silly o'clock cheese toastie/malt loaf/whatever's available and coffee and 40 winks while the coffee kicks in
I don't seem to eat som much in the night so stock up. Also I take cereal bars and jelly sweets (currently haribo tropifruit hits the spot) for in between.
I mainly drink water, when I realise I've forgotten to drink I add salt/sugar for cheapskate rehydration.
In the main though if you can do 200 under 12.5 hours and feel comfortable at the end then 300 is OK - but you need to have got past 200km by whatever time 200km/15kph gives you so you start the last 100km with min 6 hours 40 mins (pref 7 hours). Anything faster is unnecessary, I should know, ymmv.
600 you can treat as 2x300 (roughly) with a reasonable break in between - so you need to be that bit faster so you have time for that break 1 min plus however long you want to sleep - adjust leaving time by how much time you need for the rest of the ride.
400 in may awys is more difficult, as has been said. You'll definitely ride a good chunk of the night, no sleep (usually) and need to get the fiid right. I shall be using the "silly o'clock cheese toastie/malt loaf/whatever's available and coffee and 40 winks while the coffee kicks in" method this year, after 2 of 500s last year this seems to work - it's taken a while to pin down as I do at most 2 in any one year.