Back on my bike this morning - down to Cavendish and back. I actually felt ok - no signs of the hardship I had been suffering less than 48 hours earlier.
Picture of me taken on the ride by Digitalfeel:

During this morning's ride, and later in the day, there was some talk of the right training for competing in 6 hour + sportive rides.
My thoughts on it (taken from an email) follow:
For the first two things mentioned:
* To build strong muscles where they are needed - primarily legs, but also to a lesser extent core body fitness.
* Heart and lung fitness
I agree can be trained from short-term exercise. For muscle building to increase strength and power, you need to push the muscle fibres past their limits which then tiggers a response by the body for them to be repaired and their size and density increased. This can be achieved through exercise within our 1 hour timeslot.
Similarly, for heart and lung fitness, you can stress your heart (which is muscle) in the same way, and by pushing the limits of your cardiovascular system you can increase its efficiency as your body will adapt to cater for the demands. Again we can achieve this in our 1 hour slot.
However, it is the third of your list that is the issue:
* Stamina / endurance.
Looking at some basic physiology helps here. From wikipedia:
"Glycogen forms an energy reserve that can be quickly mobilized to meet a sudden need for glucose, but one that is less compact than the energy reserves of triglycerides (fat).
Glycogen debt and endurance exercise
Due to the body's inability to hold more than around 2,000 kcal of glycogen, long-distance athletes such as marathon runners, cross-country skiers, and cyclists go into glycogen debt, where almost all of the athlete's glycogen stores are depleted after long periods of exertion without enough energy consumption. This phenomenon is referred to as "hitting the wall" or "bonking". In marathon runners it normally happens around the 20 mile (32 km) point of a marathon, where around 100 kcal are spent per mile,[citation needed] depending on the size of the runner and the race course. However, it can be delayed by a carbohydrate loading before the task.
When experiencing glycogen debt, athletes often experience extreme fatigue to the point that it is difficult to move."What we suffer in our longer rides is glycogen debt. This is the only things that slows us down as our bodies efficiencies in strength and heart/lungs abilities don't depreciate.
We start off with about 2,000 kcals, but I use about 500kcals per hour when cycling (about 800-1000 when running).
So after about 4 hours I am out of glycogen. Now I can supplement this by taking more food, but even with Clive's magic gels my body can only metabolise about 250 per hour.
So I can last about 6 hours overall.
This actually ties in with my riding (and running) experience. It is that 6-10 hour period where things go wrong.
So how can this be improved?
The body also uses fat reserves for energy. This process is a lot less efficient than glycogen use, but the body holds a lot of fat (enough for days of effort).
So the key is to train your body to use it's fat reserves more efficiently, and this can be done by forcing the body into glycogen debt and carry on exercising.
The best way is to ride for 6 hours plus, as this is similar to race conditions. But of course this is hard to find the time for, even on a Sunday.
The other way is to start with as little glycogen in reserve when you start.
This can be done, and has been done by runners, through this:
Evening: go for long steady run of 2 hours (or cycle 2-4 hours)
After: Eat nothing, but re-hydrate.
Sleep.
Morning. Don't eat again and go for another long ride/run.
The theory is that you then start in the morning with lowered glycogen reserves - perhaps as low as 500kcals. So after just 1 hour of cycling or 30 mins of running, you are in glycogen debt, simulating the point that a 6 hour ride would normally put you in.
This will be a tough session, of low speeds and hardship, but will train your body to burn fat efficiently. It also an amazing way of losing weight, albeit especially taxing.
So I am thinking of doing this once a week in the run up to the etape, especially as it's getting lighter in the evenings.
I'll do a 2 hour run or ride in the evening, no food, and then a long ride in the morning (probably a EB Cavendish Ride followed by Keighley Gate/Canal into Leeds). This morning ride will likely take me 3 hours, but will be simulating the period I will hit in the Etape and White Rose between 6 and 8 hours.
Clive didn't actually think that your body can be trained to use fat very much more than it does naturally, but this approach is used by Ironman triathletes and ultra-distance runners (or so the magazines say).
I could always do the Sunday ride on an empty stomach and take no food, but that would spoil the enjoyment of the group ride, and I may get dropped towards the end of it.
I think I'm going to start from next week. So my schedule will be:
Sun - 4-5 hour quality ride
Mon - easy EB 1 hr
Tuesday - EB hilly+ extra hill reps - 1.5 hr
Wednesday - EB fast + extra hill reps
Thursday - EB hilly (optional, maybe rest)
Thursday night - 2 hour run on moor
Friday am - EB easy + moor/canal to work (fat burner)
Sat rest.
The only modification to my current training plan is the evening/morning fat-burner combo.
I'll try and throw that session in as often as I can, but not more than once a week.
We'll see how it goes.