I am not sure that any of our information in terms of specific times is going to be any help for you, if you are actually in the UK as your profile says. Endurance in the US is run by AERC, which is a whole different animal that FEI endurance. I know in Europe any 'endurance' is FEI, not totally sure if the UK is the same. But if it is, you would do best to contact someone locally about all the specifics.
Certainly the idea of doing long, slow distance and building up a base is universal.. but for us in the US, we don't have to do all the qualifying up in distance at specific times, etc that I think you need to do across the pond.