I'm struggling to find it. The closest that I have seen in my searches is from
The Science of Winning by Jan Olbrecht.
Here is an excerpt:
Muscle biopsy research has underlined the importance of the intensity as well as the duration (and thus also volume or mileage) of the training exercises for increasing the number of mitochondria (fig. 8). It was found that the peak adaptation in mitochondria content seems to occur with shorter but highly intensive training exercises. This indicates that short, intense swims are very useful to raise the number of mitochondria. Thus, not only long extensive (slow) workouts but also short, intense exercises are essential for the improvement of the aerobic energy supply process. These short intensive swims seem to affect the recruitment of mitochondria and therefore focus the training stress on this part of the biological adaptation. Long extensive workouts on the other hand, are more suitable for improving the cardiovascular system, the homeostasis and the availability of substrates, and to induce other non muscle specific adaptations.
Olbrecht, Jan (2013-12-17). The Science of Winning: Planning, Periodizing and Optimizing Swim Training (Kindle Locations 609-616). F&G Partners. Kindle Edition.
He suggests that endurance training can be optimized by "spicing up" endurance sessions with some high intensity efforts.
How can we apply these scientific findings to training practice to improve the aerobic capacity? Aerobic capacity training can only provide maximum benefit if long swims of low intensity are “spiced” with a few intensive and short stimuli. These “spices”: may only be added in restricted quantity. For swimmers with mostly slow twitch muscle fibers the quantity is strongly restricted. For swimmers who have more fast twitch fibers, the quality or frequency per week of short intense bouts may be higher. The number of short intensive bouts necessary for aerobic capacity training depends primarily on the level of anaerobic capacity and muscle force (non-invasive indication = no biopsy), but will ultimately be determined by a systematic and continuous evaluation of training effects following different doses of short intensive swims must be scheduled in the beginning of a workout and be followed by a long swim of low intensity The intensity at which the long extensive swims have to be carried out can be low since even low intensity provides enough stress on the mitochondria to provoke adaptation. In other words, the swimmer does not have to swim very fast to recruit the slow twitch muscle fibers that are responsible for most of the aerobic capacity
Olbrecht, Jan (2013-12-17). The Science of Winning: Planning, Periodizing and Optimizing Swim Training (Kindle Locations 621-631). F&G Partners. Kindle Edition.
I swear I read something very specific that the benefit of intense training had more to do with the number of "training units" that incorporated a high intensity component versus the duration or number of intervals in one session, but I can't find the reference now. I'll keep searching.
In terms of my own training I was thinking about adding a brief sprint workout at the end of some endurance sessions, maybe 3 or 4 1' to 2' intervals to experiment with the concept. Not sure though.