You are right that it's 3 intervals of 12 minutes with 8 minutes recovery between them.

The over and under refers to your SteadyState rate with is specific to the book, but based aerobic and lactate threshold rates - to get the heart rates right you should either take a MaxVO2 test or do the tests in the book

In your example you would do two intervals at the upper end of your aerobic heart rate (the SteadyState intervals) and then one at or near your lactate threshold rate (the ClimbingRepeat rate in the book).

In terms of perceived effort, that would be 2 intervals at about a 7 on a scale of 1-10 and then 1 interval at a scale of 8-9.

You are right that it's 3 intervals of 12 minutes with 8 minutes recovery between them.

The over and under refers to your SteadyState rate with is specific to the book, but based aerobic and lactate threshold rates - to get the heart rates right you should either take a MaxVO2 test or do the tests in the book

In your example you within your three intervals you do 2 minutes at the upper end of your aerobic heart rate (the SteadyState intervals) and then one at or near your lactate threshold rate (the ClimbingRepeat rate in the book). Then repeat, so that in your 12 minute interval you get 4 'over' surges.

In terms of perceived effort, that would be 2 minutes about a 7 on a scale of 1-10 and then 1 minute at a scale of 8-9.