To make this simple with three main variables in the set that you can adjust to make it more difficult or less: distance, intensity, and skill standard.
You choose the total distance of the set to be within your budget of time or energy. Then you choose the repeat lengths to be present the right amount of challenge for your weakest system.
You choose the intensity level by how you apply your effort (which determines how quickly you use up energy). Your effort could be applied to a certain stroke count (SPL) range, a certain tempo range, a pace range (a combination of SPL x Tempo), and according to the rate of perceived effort (RPE) that you intend to work at. You can raise or lower the challenge level based on which metric and which numbers you choose for that metric.
You choose what skill and what standard you will aim for or uphold during that set.
For any of these you may choose to keep it in the green where it is well within your fitness and technical capabilities to sustain, take it into the yellow zone where it is stressed a bit, into the orange where it is really challenged and you are just barely capable with good concentration, or take it just over the line to where you start to experience failure because your body is not quite adapted to that level yet.
During the practice you can set the starting challenge then change it as you go in one of these basic ways:
- Hold same distance, hold same intensity, hold skill standard
- Hold same distance, hold same intensity, increase skill standard
- Hold same intensity, hold same skill standard, increase distance
- Hold same distance, hold same skill standard, increase intensity
Notice the main principles behind these instructions:
- Only challenge certain variables in the set.
- If you are going to change something in the middle of the set, change only one variable.