Lesson 01
Introduction
Walk into any gym and “HIIT” is the headline word — short, sharp, exhausting intervals sold as the most efficient way to get fit. Walk into a serious marathon training plan and you'll see something different. The vast majority of the work is aerobic, comfortable, repeated, almost unimpressive on paper. Both methods build fitness. They build very different kinds of fitness, and they cost very different amounts to recover from.
Understanding the difference is what separates runners who progress for years from runners who train hard for six weeks, get injured, and start over.
Lesson 02
Why this matters
- Marathon performance is overwhelmingly aerobic — somewhere around 99% of the energy you'll use on race day comes from your aerobic system.
- HIIT improves traits that matter for short, repeated efforts. Marathon races punish runners who optimise only for that.
- Recovery cost is wildly different. A true HIIT session loads the nervous system hard. An easy aerobic run loads it gently — which means you can do it more often, which is what produces marathon fitness.
- Knowing when (and how rarely) to use HIIT inside a marathon block protects you from the most common cause of failed cycles: too much intensity, too little base.
Lesson 03
How the method works
Aerobic training
Mostly easy and steady running where you could speak in full sentences. Long, repeatable, low-stress. This is where heart, capillaries, mitochondria, tendons and bone adapt — the structures the marathon needs.
Threshold / tempo work
Sustained efforts you could just about hold for around an hour. Sits above easy aerobic but below “hard.” This is the most direct bridge between easy running and marathon performance.
HIIT (high-intensity intervals)
Short, hard repeats with full recoveries — usually 20 seconds to 4 minutes of work, often near 5K effort or harder. Excellent for raising the aerobic ceiling and improving running economy, when used sparingly.
How they combine
A healthy marathon week has a foundation of easy aerobic running (often 70–85% of the volume), one or two quality sessions (tempo or intervals), one long run, and optional strides. HIIT, when used, sits inside that quality slot — not on top of it.
Lesson 04
What happens physiologically
- Easy aerobic running grows heart stroke volume, capillary density and mitochondria — the deep machinery that lets you keep running for hours.
- Threshold work improves lactate clearance — the same speed feels easier because your body deals with the by-products more efficiently.
- HIIT improves VO2 max and running economy quickly, but the gains plateau in 6–10 weeks for most runners.
- Heavy intensity loads your sympathetic nervous system. Too much, too often, and sleep, mood and easy-pace heart rate quietly deteriorate before any specific injury appears.
Lesson 05
Real runner application
- Beginners and returners: build aerobic base first. Quality sessions can wait until you can comfortably run 4–5 times a week.
- Intermediate marathoners: one quality session per week is usually enough. Save HIIT-style intervals for short blocks (3–5 weeks) when the base is solid.
- Time-poor runners: short HIIT can preserve fitness in a busy week — but never as the only training type if you're targeting a marathon.
- Returning from injury: re-introduce intensity last, not first. Easy aerobic miles rebuild the structures HIIT will then load.
Lesson 06
Common mistakes
- Confusing “hard” with “productive.” Adding intensity to a tired week usually slows fitness, not speeds it.
- Running every easy day too hard, then having no headroom to do real intervals.
- Stacking HIIT, tempo, long run and strength inside the same week and wondering why fitness plateaus.
- Using HIIT as the only training type while training for a marathon. The race is too long and too aerobic for that to work.
Lesson 07
What beginners often misunderstand
- “If I'm not gasping, it isn't training” is wrong for marathon prep. Most of the adaptation happens at speeds that feel almost too easy.
- Elite marathoners run a lot of slow miles — not because they're holding back, but because that's where the depth lives.
- Two hard sessions a week is plenty. Three is usually too many. Four is reliably how injuries arrive.
Coach insight
The runners who improve year over year are almost never the ones who train the hardest. They are the ones who train the most aerobic, most consistent, lowest-cost week they can repeat for months.
Recovery layer
Recovery considerations
- After any HIIT session, plan an easy day or rest the next day — non-negotiable in a marathon block.
- Sleep, not supplements, is the biggest recovery lever after intensity.
- If your easy pace heart rate climbs week over week or sleep gets worse, lower intensity volume before lowering total mileage.
Judgement layer
When NOT to use this method
- In the first 4–8 weeks of a marathon block, when aerobic base still needs to be built.
- In the final 2 weeks of taper — short sharper efforts are fine, but full HIIT sessions are not.
- In high-stress life weeks (poor sleep, illness, travel) where the cost will outweigh the benefit.
- When the goal is to extend your longest run — endurance is built by repetition, not intensity.
Practical layer
Practical examples
Aerobic-led week (marathon build)
5 easy runs of 30–60 min, one tempo session (e.g. 20 min at “comfortably hard”), one long run of 90–150 min. Optional: 4×20s strides after one easy run.
Short HIIT block (mid-cycle, 3–5 weeks max)
One session per week: e.g. 6×3 min at 5K effort with 2–3 min easy jog between. Plus one tempo every other week and the long run weekly.
Busy week (maintenance, not progression)
3–4 easy runs of 30 min plus one short threshold workout (e.g. 4×5 min steady). HIIT is not a substitute for the long run in a marathon plan.
Marathon application
The marathon rewards depth, not flash. Aerobic base is what carries you through the second half of the race. HIIT is a finishing tool — used wisely, it sharpens. Used recklessly, it breaks the block.
Confidence note
If your training feels too easy most days, you're probably doing it right. The work that produces marathon fitness rarely feels heroic.
Journey Coach
Apply this lesson to your week — your level, your race, your life.