Most LDWs I've met have agreed you get to a place of waking up and not wanting to keep walking sometimes, but by that afternoon you may feel the entire opposite. I'd say the most common reason for abandoning a LDW (other than injury), is actually caused by poor nutrition. People who don't bring enough with them, end up hiking on empty from day 3, and aren't proactive enough to really up the amount they put in to help repair/fuel them better.
I have to say that doesn't really match my experience. I've usually found that on waking up I'm raring to go, and it's only once I've slogged my way through 3 feet of snow or a morning of heavy rain that I'm beginning to wonder what I'm doing.
As to malnutrition, again I have reservations. There are very few LDPs that don't have supply centres of some sort, and the ones that don't usually mean that you don't have anywhere to go but onwards anyway. I did do one LDP where we ended up having to camp for a night at an unexpected location and very little to eat (a pack of nuts and raisins and a Yorkie bar between us I seem to remember). The next day we continued and about lunchtime headed for a pub marked on the map where I downed a steak and chips and my companion a huge plate of pasta. Batteries recharged.
The fact is that I've backed out of two LDPs in my life. The first was, as suggested in my first post on this subject, poor weather. We'd walked the previous day in appalling weather conditions, and woke up to more, and the prospect of over 20 mountain miles with cloud cover at about 500m. We talked about it and felt that there was little point in trekking that distance surrounded by thick cloud which would mean we couldn't see anything. In other words a day of my precious 'leisure time' spent walking for virtually no enjoyment. Ok we could have gone to walkers' bars boasting about how we'd done this, that and the other, but that doesn't appeal to me at all, whereas making good use of my leisure time does.
My second 'retirement' was on a relatively short LDP called the 'Six Shropshire Summits'. My then partner wanted to visit some friends in Shrewsbury, so we agreed that she would do so while I did the SSS. We agreed I'd spend a night and we'd meet up half way at Long Stretton and that we'd all have a few beers and a meal in Shrewsbury on the second night. The walk went off to a very bad start. It was at the time of one of the 'Mad Cows' Disease' panics, and the area around the first summit was quarantined so I did what I could that day and met up with my partner as arranged. I set off the following morning but had an awful time negotiating the route because thanks to the Mad Cows Disease restrictions, the local farmers had simply allowed the footpaths to get swallowed up by the summer vegetation. I battled on via whatever routes I could achieve until I finally reached a footpath that went through a gap in a very high hedge which was totally impassable without a machete. Consulting the map showed, on this occasion, a very long detour with mostly uninspiring road work. I then calculated that if this level of alternative route finding kept up there was no way I'd be able to meet up later as promised. I'm not the henpecked type but I'd never have heard the last of it if I hadn't turned up for the evening rendezvous (and she was driving me home anyway) so I found a pub near a railway station, had a couple of pints and threw in the towel. There was no way I could have completed the walk anyway since I'd missed the first summit due to restrictions. What this came down to was that I'd been forced to spend far too much walking time on roads and wasn't really enjoying it that much, so once again, it was proving to be poor use of my leisure time.
So from my point of view, I find I'm fresh and ready to go first thing in the morning, and malnutrition has definitely never been a factor. One thing I would say that I believe helps a lot with LDPs, is to have a very light day on the third day. The first day is when you find out if you were fit enough in the first place, the second day could be difficult but if you know the 3rd day is a 'day off', then you have that to console yourself with. After the 'day off' I find that my body has got used to the work effort, and has recharged with the easy day so I'm then ready for anything. I would therefore suggest that rather than malnutrition, if people do simply 'give up' it's more likely to be because they've pushed their bodies too far with insufficient recovery time.
S