Interval running is the highest-leverage cardio format most runners do wrong. Not wrong as in dangerous, but wrong as in unstructured: run hard until it hurts, jog until it stops hurting, repeat until bored. That session feels productive because it is uncomfortable, but discomfort is not a training variable. Structure is. The whole value of intervals comes from controlling three numbers: how long you work, how long you rest, and how many times you repeat it.

The anatomy of an interval session

Every interval workout is the same machine with different settings:

Work segment. The hard effort. Defined by duration (say 2 minutes) or distance (say 400 m), at a target intensity. Heart rate zone is the most practical intensity target for most runners, because pace targets collapse on hills, wind, and bad days while zones self-correct.
Rest segment. The recovery between efforts, jogging or walking. Its length relative to work is the work/rest ratio, and it is the most misunderstood setting on the machine.
Rounds. How many work+rest cycles you complete. This is usually the safest variable to progress first.

A session is then just notation: 6 rounds of 2 minutes work at zone 4 with 90 seconds rest reads as 6 x (2:00 W / 1:30 R). If you cannot write your interval session in that form, you do not have a session, you have a mood.

Choosing a work/rest ratio

The ratio controls what the session trains:

1:2 or easier (rest twice the work). Full-ish recovery between efforts, so each work segment can be fast and clean. This is where to start, and where speed-focused sessions stay.
1:1. The standard aerobic-power middle ground: 2 minutes on, 2 minutes off. Efforts stay strong but recovery is never complete, so the cardiovascular load accumulates across rounds.
2:1 or harder (work twice the rest). Incomplete recovery on purpose. Late rounds are run on residual fatigue, which is the point, and also why this belongs at the end of a progression rather than the start.

The honest starting prescription for someone new to structured intervals: 4 to 6 rounds of 1 to 2 minutes work in zone 4, with rest at 1:1 or 1:2, once a week. Boring on paper, effective in legs.

Progress one variable at a time

Interval progression fails in a predictable way: everything gets harder at once. More rounds and longer work and shorter rest and a faster target, all in the same week, and by week three the session is unfinishable and gets abandoned.

The fix is the same rule strength training already taught you: progressive overload means moving one variable while holding the rest constant.

Add rounds first. 4 x (2:00/2:00) becomes 5, then 6. Cheapest to recover from, easiest to judge.
Then lengthen work. 6 x 2:00 becomes 6 x 2:30 at the same zone target.
Then tighten rest. Move from 1:2 toward 1:1, and only past that if the session's goal genuinely demands it.
Intensity moves last. Zone targets creep up only when the structure above is stable at the current one.

And roughly every fourth week, take the volume back down. Intervals accumulate fatigue quietly, and the deload logic that applies to your lifting applies here identically.

Judge the session by its numbers, not its suffering

The trap with intervals is grading them by how destroyed you feel. Two measurements do a much better job:

Zone compliance. What fraction of each work segment was actually spent in the target zone? Sprinting the first 20 seconds and fading for the rest shows up as poor compliance even when the average looks fine. Rest compliance matters too: if your heart rate never drops toward the rest target, the rest is too short for you today.
Round-to-round consistency. Compare pace and heart rate across rounds. Round 1 and round 6 within a few percent of each other means the session was set correctly. A cliff after round 3 means too fast, too little rest, or too many rounds, in that order of likelihood.

This is exactly the kind of bookkeeping software should do while you just run. In Zirv, an interval session is guided: the app tells you when to work and when to rest, coaches against your target heart rate zone during each segment, and counts the rounds, while the watch tracks your heart rate alongside. Afterwards it reports zone compliance plus per-round work heart rate, pace, and distance, with rest-segment recovery next to it. Week over week, progression stops being a feeling and becomes a comparison: same session, one variable moved, numbers side by side.

A four-week template to steal

Week 1: 5 x (2:00 work, zone 4 / 2:00 rest)
Week 2: 6 x (2:00 / 2:00)
Week 3: 6 x (2:30 / 2:00)
Week 4: 4 x (2:00 / 2:00), deliberately easy, then start the next block from week 2's level

Run it after a 10 minute easy warm-up, judge each session on compliance and consistency, and only move a variable when the previous week's numbers held together. That is the entire method: intervals progress like lifts do, one controlled change at a time, measured instead of guessed.