HIIT is not supposed to be every day

Intensity is a budget

You do not have unlimited hard days. Two, maybe three a week — that is the budget. Spend more than that and you are not training harder, you are just training tired, and tired training produces worse sessions at every intensity.

What a sensible week looks like

• Two hard days. Genuine HIIT, with genuine rest intervals.
• Two or three easy days. Aerobics, a walk, mobility work. These are not filler — they are what lets the hard days be hard.
• One or two off days. Actually off.

The tell

If your hard sessions are getting slower week over week, you are not under-training, you are under-recovering. The fix is counterintuitive: take a day off and the next session will be better than the last three.

Hard days work because easy days exist. Remove the easy days and the hard days stop working.