"Sandwich" workouts - where do they fit?

I’m a triathlete, training for Ironman distance.
I stumbled upon workouts like Arrow, Mono, Charlotte, Izaak Walton, Vennacher, Three Sisters, Wheel etc. They are basically Threshold or Over-under intervals, “sandwitched” by endurance work.
They seem very fitting for long distance training, but I can’t find them included in any training plan. I’m wondering what the general opinion about these kind of workouts is; who are they suited for?
I also found that they cap at 3x12 over-under intervals and don’t go beyond that. Why is that?
I’m thinking about creating similar workouts but with longer Threshold or Over-Under durations between.

I use a lot of these (in fact nearly all of them) in the Saturday slot for o/u in my own hybrid MV base block 1 when I do 75mins SS Tue Wed Thr / Zone 2 Friday - one of these Saturday and then SS for 90-120 mins Sunday. I then insert standard o/u like McAdie/Palisade/Tioga in base 2. My thinking is that some of the o/u are too hard for November when UK TT starts in about April…or later with Covid . So I think they still give some intensity without killing you and you get endurance as well - win win! I don’t think that we need quite that level of pain early in the off season so far from races. My thoughts but works ok for me…and if you are a long course triathlete it should be good - plus I recommend Castle peak and its variants as well - bit longer for some additional er…fun :laughing:

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Agreed. I can’t imagine doing vo2, O/U, threshold work in winter when the season is so far away. SSBHV is tough but swapping SS days for z2, particularly outdoors, breaks it up. Supra-threshold work gets me to a peak pretty quickly so trying to hold that for months on end would be tough, not to mention constantly going to the well months away from racing.

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I completed Wheel yesterday and I did Senger today. I have to say I really like these kind of workouts, it’s a very nice mix of endurance and hard work!