Hi,
This is on the to do list, but did manage to walk the Pennine Way (N to S) last year in 10 days so have some idea what is entailed.
If you are carrying all your stuff then you need to travel light: as much as walking boots are better, I felt more comforatble in trail shoes. If you are using a baggage service then take both and alternate, certainly helps with wear and tear on the feet.
On wild camping - yeah its questionable as to leaglity, but saw a number of wild campers and like cragchick says if you're out the way you won't get noticed/moved on. Did stay a couple of nights in a YHA, and found it fine. Was so knackered I could have slept thru any amount of noise.
Planning is also the most important thing: we drew up a schedule and barring one day when we had to cut short by 4 miles (which made the next 4miles longer..) but add in contingency plans and 'get outs' if need be. We used offical guides , took the Wainwright books, a compass, and I like the Trailblazer series. Think they do have GPS signalling now, but to me thats cheating. You want to be on a mountain side with the o/s blowing away and trying to get a compass out the bags with soaked hands...
Training wise (I've done about 15 marathons) I just did loat of miles, trying to get out every day with back to back longer runs at the weekend, averaged about 50 miles a week I reckon.
Enjoy it!