I promised some tips about long-distance trail walking. They all apply to walking for several days on a trail or not. Some may seem obvious, but on the West Highland Way last year I came across people who'd thought the WHW was a good opportunity to try out a new pair of boots and others who'd clearly not found out what a full pack felt like on their back until they set out for the Glasgow train.
PLAN. Use the on-line information. Book accomodation in advance unless there's an abundance of it on the way (as on MOST parts of the South-west Coastal Footpath). Identify places where you have no alternative - just one place to stay - and book there first, adjusting your dates to fit what they've got. Make a list of what you need. Seek maps and guide-books in good time: many bookshops will have to order them. Take into account that the steepest climbs may take two or three times as long as mildly hilly walks - and that while coastal walks may have no mountains, you may have to keep going steep down, across half a mile of loose sand or shingle and then steep up again. Think about what you're asking of yourself. Can you really do an average 20 miles a day? What's your evidence? If you can do 25 miles in hilly but not mountainous country with full pack but that's near your limit, is there any way of breaking up that three-day 25 mile - 23 mile - 26 mile sequence? I fell down on that count doing the Wye Valley Walk - but oddly, the toughest day was the first of the three!
PRACTICE. If possible, not only do all the long walking you can in the months leading up, but take a short break to do intensive walking in roughly similar country about two weeks before you start the real thing, carrying roughly the same weight in your pack and using the same key equipment - boots, pack/rucksack and so on. This is a good time to find that those broken in but newish walking boots are more comfortable with the top toggle left unused or that the combination of fleece and cagoule doesn't work because the cagoule doesn't cover the bottom of the fleece. Do at least three-and-a-half days' intensive walking (but enjoy it too). Make it tough: the first time I did the WHW, I'd trained in the Lake District and there was nothing on the WHW as tough as ascending Blencathra or going over Black Sail Pass. That made me feel good on the WHW! Make sure you know from experience how to use your compass, too, or GPS if you use that.
CUT DOWN ON WEIGHT (unless you're using sherpa services). Think. Most clothes can be washed and dried somewhere on the trip, even if the washing is in a wash-basin and the drying by hanging from the top of your pack. How many pairs of each item do you really need? On any trip of less than 200 miles, I'd suggest you can plan to wear each pair of socks four days (two days, one wash, two more days' wear), but you shouldn't need more than three shirts. Do you need a tube of cream for insect bites, or will a small tube of antiseptic do? Leave the thermos behind: you don't need hot drinks on the walk and it's extra weight. Accept that the only reading matter will be the guide-book and anything you can pick up where you stay and then leave there, plus a newspaper occasionally.
YOUR FEET ARE THE MOST IMPORTANT THING. Despite what I said above, don't skimp on supplies of plasters and the like. A spare pair of foam inner-soles is advisable. Carry a support bandage: if you get a muscular strain, this can make the difference between a huge disappointment and success.
TAKE MAPS. OK the guide-book probably has maps, but they'll show only a narrow band along the route. That's not enough if you go off-course or if you're trying to orient from distant landmarks. Doing without a guide-book is possible, but tends to trip you up on small details, like where to go when the path enters a built-up area or where precisely to cross the river.
CARRY AS LITTLE FOOD AS POSSIBLE. Eat a good breakfast, make do with, say, a handful of raisins, a bit of chocolate and a plum at midday, and eat a good, balanced meal in the evening. The balance is important because on the longer trails like the Pennine Way if you're not careful you'll develop dietary deficiencies which will affect your strength and resilience.
BE BLOODY-MINDED. Rain is NOT going to stop you. That crap guide-book misled you and you're three miles adrift on a day when you have 24 miles to walk? Let your anger propel you until you realise you're reasonably on your time target again. But don't ignore problems like a slight pain in your foot. They'll get worse and there's probably something you can do.
HAVE FUN. Stand and stare at the water flowing under the bridge. Stay in pubs with good beer and company. Celebrate when you finish. It may sound grim if you haven't tried it, but it's huge fun and the feeling at the finish is great.