In reply to bluebus:
I have successfully completed several multi-day walks GR20, GR221, John Muir Trail etc. However:
I did the first 10 stages (as described in the Cicerone guide) three years ago and quit for various reasons (excessive rain, blisters, too much kit, motivation problems). We usually took well over guide book suggested times which shouldn't matter but somehow it does IYSWIM. Some initial fitness makes sense, otherwise the first few days will be (too) painful. I am over 60 and for me carrying as little kit as possible is essential. So some multi day walks with your chosen kit beforehand is a must really (and then fine tune it accordingly)
It can rain a lot especially nearer the Atlantic and it can get very hot. The first two stages as described by Cicerone are too long unless you are super fit to start with (30 & 31K). Wild camping where necessary gets round this problem. (Are you planning on taking full camping gear?)
It is certainly tougher in my opinion than most UK walks e.g. the Pennine Way, and you need the right mental attitude to think about walking for 40 plus days.
Having said all that I am seriously thinking of resuming next year, I hate unfinished business.....
p.m me if you want any more info.