Remember you asked for this! Based on what you have done, you don't need my help. If I come across as anything other than trying to be helpful, I apologize. These are just my opinions.
In my opinion there are two things you have to train for. One, the cold. Two, being able to swim the same speed for a long time. I think your question is about the swimming, so that is where I will focus.
I don't know if you have read: Gold in the Water. But, I recommend it. It is the story about Santa Clara Swim Club swimmers preparing for the Sydney 2000 Olympics. One key take-away for me was the coach's focus on a repeated standard set. In today's world, coaches spend a lot of time trying to be creative - trying to keep the swimmer from becoming bored. (I guess it the entertainment age.) They come up all kinds of workouts that never repeat the same set. This coach believed that you had to have a standard set to truly measure progress. The clock doesn't lie.
Since I live in an area that doing EC-like swims are completely unnatural to people, I work out alone. As you may have experienced in life, it is easyto fool yourself. I believe creative sets allow the truth to hide. The clock doesn't lie.
At some point I learned that, because of the dramatic tide changes The Channel, swimming the EC is nothing like almost every other swim. The captain has to guess/ estimate the best time and place for you to leave the UK. He makes his decisions based on what he believes you can do. (On this swim, I was about 15-20 minutes late to a spot and it cost me about two and a half more hours of swimming. On my failed attempt in 2012, my time was dead on target. I did a straight-line measured 18 miles (French inter-coastal) in exactly 9 hours (I then hit the tide coming against me at 2mph and was stopped at 12 and a half hours just a half mile from France.) I believe that on the first swim the captain, for understandable reasons, underestimated me and we hit the flow off of France three hours too early and on the second swim (the "successful" swim), it was my fault for missing by 15-20 minutes.)
I also believe that you are training for the end of the swim, not the beginning. But there is a risk at the beginning of going out too fast:
Swimming too hard because of the adrenaline. I don't think most people can keep pumping adrenaline for 10 plus hours.
So, for these reasons, I aim for consistency above everything else. I haven't swum fast, or even tried to swim fast, in four years. I have worked hard to make my muscle memory go just one speed. If my arms are moving, almost no matter how tried or how fresh I am, I am doing the same time.
So, with all that said my workouts are very, very simple. If I am in a 25 yard pool, I do 100's on the 1:40 for however much time I have. When I started this journey four years ago, it was just 15 at a time. Now, I rarely bother getting in the pool unless I have time to do 70. My minimum on Saturdays is 100. I have done 200 a couple of times and 150 is not uncommon. It is easy to keep track because three 100's at 1:40 is five minutes. So, if you can see a regular clock while swimming and you know when you started, it is easy to check your count. If you can't see a regular clock, you can use the pace clock to do the same thing. The clock doesn't lie.
I try to do between 1:25 and 1:30. Just as the coach in Santa Clara wanted to get feedback on progress, I get immediate feedback. The clock doesn't lie. I know immediately if I dogged it.
I don't go hours at too slow of a pace before I figure out I messed up. If I go slower than 1:30, the next one is faster. I pick it up.
If I go faster than 1:25, I think that is bad too. It probably means that I let myself get caught up in someone else's swim in the next lane. The only thing I should care about is the clock. If I go under 1:25, the next one is slower.
It also creates mental and emotional toughness. When you swim alone you don't get the reinforcement that others get in team workouts - "Hey Joe did the EC. I can keep up with him in workouts; I bet I can do it too." Doing a lot of the same thing at the same speed, gives me the reinforcement that I need swimming alone. And as you get tired, it is hard to make yourself go again. Making yourself go is an important emotion to develop, I believe, to keep your feeds short. You're in the habit of only a 10-15 second stop.
(Oh, BTW, always start your swims from the deep end - even if you are the only one in the whole pool doing it.)
In a 50 meter pool, I do straight swims and I try to keep pace at 3,400 to 3,500 per hour. I wear a watch that I check constantly. Yesterday (8/30) I did a real swim for the first time since the EC and I did exactly 7,000m at 1:59 minutes.
Even as simple as these workouts are, I keep a swim log. It helps me as I get closer to a swim to review my swim log and remember how much I have done, how long I have gone and how consistent I have been. (My swim log doesn't lie either.)
Back to the point about how I believe you train for the end of the marathon swim, not the beginning. in my workouts, I try to make things as mean and nasty as I can as soon as I can. This is the complete opposite from what you hear from most coaches. If I am doing 10,000m or less, I never hydrate. (And I never stop to stretch. It takes focus not to let yourself stretch in those 10-15 seconds between hundreds.) At the end of a marathon swim, your body isn't processing new fuels as well as it does normally. I try to simulate that the by not hydrating. In fact, I typically don't eat or drink anything after 8pm on Fridays so that I am doing my long swim on Saturday mornings with nothing in me.
However, I bring lots of things with me and the minute my swim is over (right on the pool deck) I start rehydrating, eating, refueling.
(Also, I have learned to stretch while swimming - hyper-extending, swimming through leg cramps, etc.)
If I haven't already sounded like a goofball to you. I am often asked if I swim every day. I do not. I have a family, work, etc. So, it is tough to find swim time. I think it is a lot more important to do long swims than frequent swims. I don't swim unless I can get at least two hours, because I work at getting comfortable working through what I call "pain thresholds" or levels of pain. In straight a swim, I go through a certain level of pain between 3,000m and 3,500 and another around 7,000 to 8,000. (After that I am
fighting fatigue, not pain thresholds.) For me, I think it is important to swim through those pain levels without slowing down. I don't think swimming for an hour does me any good at all (to prepare for a marathon swim), so if that is all the time I have, I use it for work or spend it with family.
Again, hope this didn't put you off. It may be all wrong. It is just what I think and what I do. (Hope you aren't laughing at me!)