I've done some of these. At my fittest, I was fit enough to be doing a few martial arts classes a week but wasn't exactly superfit. I did a few long training walks before my first one (longest was 20 miles), more to reassure myself than anything. The training plans they have for these walks are usually really good though and more than enough to get you ready. For me, the main things are:
Walk enough to properly break in a really good pair of walking boots/shoes, don't try and do it in fashion trainers or wellies!
Get to know how much water you need to keep you going without dehydrating OR needing the loo every hour, and work out which energy drinks work for you without making you feel sick or giving you indigestion.
Likewise snacks - different things 'fuel' different people particularly well. Work out what gives you the most miles for the fewest mouthfuls, just so you don't have to carry loads with you.
Also, figure our how many layers you need to wear, and what type. I like gym leggings (with pockets, and not the stupid push-up scrunchybum ones) for walking, partly because they dry out easily if they get rained on, but they don't suit everyone. Also I don't get cold easily usually (if anything I overheat easily and I don't want to have to carry anything I have to take off), but I definitely feel the cold more when I start getting tired so I'll have a couple of thin, light long-sleeved tops in my rucksack.
Spare socks - take several pairs and use the rest breaks as a chance to put on fresh ones. At the same time check your feet over for 'hot spots' - you'll feel these before blisters start forming and it'll give you a chance to apply plasters before the damage is done.
A light, effective headtorch or one of those lumilight hat things - the routes aren't always lit towards the end and you're tired and more likely to stumble.