I came nowhere near completing the CF, even though I'm in reasonable shape for my age (66), a life-long weight-trainer and had spent months training on hills etc. I had to bail by Logroño, 7 days in.
Unexpected, adverse weather in the earliest days was a HUGE factor. The day before I arrived at SJPdP, three pilgrims died of dehydration on Camino routes. The day of arrival, the folks in the pilgrim's office were advising people that the weather forecast for the next day was extremely poor for hiking (cold, very heavy rains and low cloud deck), and to either abort or use the Valcarlos route.
Too many did neither. People were arriving in Roncesvalles hypothermic, and others had to be medivac'ed off the mountain due to injuries from the extremely wet/slippery conditions. Many had buggered up their knees or twisted their ankles. Most of what I saw wasn't particularly well-correlated with age or experience, either. (There were middle-aged couples who were sailing through, and hikers who had thru-hiked the Pacific Crest Trail who were already in trouble, and who were done by the time they got to Puente la Reina.) The next day, the descent into Zubiri (which was still quite wet from the localized heavy rains) all-but-finished a number of other
peregrinos, though some just aborted at the Alto de Erro staging area and got a taxi into Zubiri. I met several
peregrinos later in Larrasoaña who looked like the walking wounded, and these were people who had walked the CF before.
By day 6 at Los Arcos, I saw a number of familiar faces who were bailing, mostly because the injuries were piling up and they didn't have the luxury of staying in one place for days to try and reset (their itineraries wouldn't allow it). I made the hard decision at that time to be one of the bailees.
The Camino provides, but what it mostly provides is the unexpected and a humbling experience. I'm glad I went, I'm probably going back next year to try again, and I really,
really believe that the "standard" hiking itinerary popularized by people such as
John Brierley (RIP) is
far too ambitious for the early stages, particularly because the end of each day is a somewhat steep downhill which becomes much worse in poor weather and which is bad enough even when conditions are favourable, because you're tired and clumsy from the hike. Those downhills should be at or near the beginning of a stage, not the end! If it's not possible for them to be at the beginning, then the stage leading up to them should be as short as possible.
I have my candidate "re-built" itinerary already in mind, which features a gradual ramping-up of effort over the first days until Puente la Reina, and would take at least 6 days (and probably 7, weather permitting) to make it to PlaR, after which the distances gradually "open up."
What I've said above will probably appall many of the hard-core contributors, but if someone wants to know how to be successful on the CF, they should probably consider why some folks "failed & bailed." That's as instructive as the successes.