I can't take all the credit on this one. I attended a lecture at the USA Swimming headquarters here in Colorado Springs a couple weeks ago. There were a few national team athletes there, as well as coaches. Great Q&A after the lecture, and the description of this set sounded awesome.
They called them "aerobic sprints". You'll see what I mean........
This set is SCY, yields 3,350 yards, and takes about 1:15 to complete.
8 x 75 on 1:15
8 x 100 on 1:35 800
Descend 1-4, 5-8
Main set: Aerobic sprints
2 x (15 x 25)
Easy 100 between each round.
The goal of the main set is to get :15+ seconds rest - almost allowing you to stay aerobic (almost). So it is more of a threshold set, but the athletes need to work for their rest. Change the send-off, of course, to accommodate your athletes. Again, you're targeting :15-:20 rest (max) here. So if they can hold :15 25's then give a send-off of :35. If you want to make them really work, make the second round send-off :30.
Truth be told, I had to really shave down the send-off for a few of my elite triathletes. :30 was too easy as they're anaerobic in nature. Cutting that to :25 helped.