I responded "knee/Achilles" because that was the main cause for me stopping.
I've had problems with my knees since May, and every time I ride over about 300km they get a little bit grumpy with me - I thought I could sort it out with gentler riding and so on, and I thought I had it under control - I rode 250km in June and felt pretty awesome, and I thought I was over it. I then tried to ride a 400km about 4 weeks out from the start as a final training ride, and only made it 100km before I climbed on a train and went home, pretty despondent.
At this point, it was too late to back out and get my place reallocated, so I decided I would line up on the start and see how it all went - knowing it was likely my knees would give up, potentially even before the first control.
I lined up on the Sunday at 1215, rode the first day up to Louth, slept on a bit of floor there, had no breakfast and generally had a bit of a miserable time, but pressed on, I was one of the people who stripped the poor shop at Barton upon Humber clean of food at about 0600 on Monday morning, pressed on to pocklington and enjoyed the control at Coxwold, saw my parents very quickly at thirsk as we bounced through and then on to B-Castle and Brampton for the night - I got one of the last beds that evening in Brampton, and was very grateful for it - my knees hurt each night as I slept and took quite some time to get going in the mornings. on Tuesday, I rode to Edinburgh and back, not going to lie - getting into Edinburgh (with a mechanical - which is the topic of another thread) I was pretty emotional, and pretty amazed that I had made it that far. I got back to Brampton that night at 0230 - having lost 2.5hrs sorting the mechanical - slept ok, knees still hurt a bit, but nothing show stopper at this point.
Wednesday morning, and I actually felt pretty bloody good - got moving stormed over Yad Moss, and into B-Castle, on to Thirsk, pick up supplies from my parents and pressed on to Pocklington, by the time the Howardian Hills came round my knees were not happy, and I was struggling to keep up with the group, by the time I got to Pocklington, I was finding it hard to keep the bike moving without my knees hurting so much I wanted to vomit - in particular, the transition from free wheeling to pedalling was absolute agony. I decided to sleep on it, and try them in the morning. I tried riding round the car park at about 0415 in the morning, I really couldn't do it - it was still agony to transition from pedalling to free wheeling and it gave me that sick feeling to do it. I decided that whilst I might be able to press on to Louth, maybe even further - there wasn't 350km left in my knees on that ride - so fearing that I would do permanent damage - I got on the train at York and went to Leeds to sleep before heading back to London.
On a set of knees that couldn't do 150km 10th July, I did 1100km all told (including the ride of shame to the train station). I'm overall happy with what I got done, I still had a minuscule amount of time in hand when I packed (due to sleeping). My next step is saving up some cash for a bike fit, and training for PBP. I aim to be back for 2021, when I can ride it as my 40th birthday present to myself.