If reference to Gav's dynamic breathholds --

When I first started working on this I came up with doing a set of 10 slow 25s on a flexible interval, not based on how I felt because I would have stopped doing them after a few if it were based on feeling. I would take whatever amount of time I was underwater, double it and add :10-:25 seconds so that I was leaving on the quarter minute of the pace clock (easier to keep time that way).

Example:
1-- down :25 double it :50 + :10 puts me on the :60
2-- down :32 double it 1:04 + :11 puts me on the :15
3-- down :38 double it 1:16 + :14 puts me on the :45
4-- down :41 double it 1:22 + :23 puts me on the :30
etc

First time I struggled to come close to a minute at the end of the 10th one, eventually I worked up to over 2:30 and then decided to try my first 75 underwater, since age-group swimming 20+ years ago, and it was easy. From start to 75 took me about a month, maybe a month and a half, going 3 times a week.

As with all perceived limits, it's a whole lot easier if you can take your mind off the big picture and just let it happen at whatever pace it takes you. I found this a good way to make the big task easy. It wasn't hard to say I want to stray down :05 longer than the previous one. And I found in my play I started making more 2nd and 3rd efforts.