Things I learnt when I did Amberley to Eastbourne and back in a day.
Bike position – bearable for 6hrs is not good for 10-12
It is not possible to ride
it all in Z2 the hills will have you at threshold
Don’t descend unless you KNOW you are on course
Be unfashionable and have a hugely low gear for late in the day
Eat and drink a lot but only stuff you have already trained with.