Training endurance is like building a sand castle. You need sand and water, the sense not to overdo it, and the expertise to make something great.

  • Basic endurance is like piling dry sand. It can’t be shaped, just piled higher. For every inch of height, it takes a greater volume of dry sand than the inch before.
  • Adding intensity is like adding water. You can increase the angle of repose of the sand, steepening the cone, getting higher with the same volume.
  • Artists (great coaches) can get the saturation just right, creating amazing sculptures. But those sculptures will always be temporary.
  • HIIT proponents misunderstand the sand/water relationship. They assume that water is the big secret:
    • “If water makes sand piles steeper, let’s just use that!”; and
    • “If the pile of wet sand starts to crumble, add more water!”
  • The distinctions are:
    • First, it’s not either sand or water, but the mix of sand and water that’s important;
    • Second, wet sand is born from dry sand; it’s not independent. The volume of dry sand still determines the ultimate height of the cone or size of the castle; and
    • Third, if the pile starts to ooze downward, adding more water won’t help. To restore it, you have to dry it out (by adding more dry sand).

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