TL;DR
- Many runners accidentally live in a "moderate" zone where workouts feel hard but do not produce great adaptation.
- A useful rule of thumb is 80% easy, 20% hard (measured by time or sessions depending on your training).
- You do not need perfect zones. You need consistent definitions and a weekly review.
- Apple Watch does not show a full intensity distribution dashboard, but you can approximate it via Strava or your training platform.
- Century helps you combine intensity distribution with recovery signals so you keep improving without digging a hole.
What "training zone balance" actually means
Your watch can show you a lot of numbers, but the big picture is simple:
- How much time did you spend at low intensity?
- How much time did you spend at moderate and high intensity?
When that balance drifts toward "too much medium-hard", runners often get:
- persistent soreness
- plateaued fitness
- sleep quality dips
- more illness and niggles
Runner's World recently highlighted "training zone balance" (also called training load focus or intensity distribution on some platforms) as an underrated metric because it makes hidden patterns obvious.
The 80/20 idea in plain English
The 80/20 concept is not a religion. It is a guardrail.
A simple version:
- Easy: conversational, you could keep going, breathing controlled.
- Hard: intervals, hills, tempo blocks, races.
If your week is all "kind of hard", you are not really doing either category well.
The win is not in the exact percentage. The win is in the separation:
- easy days are easy enough to recover
- hard days are hard enough to stimulate improvement
Step 1: make your zone definitions consistent
If you use heart rate zones, start with something pragmatic:
- Use the zones your watch already has, but confirm the max heart rate or threshold setting is not wildly wrong.
- Then keep the settings stable for 3-4 weeks.
If your watch zones are a bit off, do not panic. Over time, trends still matter.
Step 2: measure your week in a way you will repeat
Pick one method and stick to it.
Option A: simple session counting
For many runners:
- 1-2 hard sessions per week
- the rest easy
Example (5 runs per week):
- 1 interval session
- 1 longer run with some steady effort or hills
- 3 easy runs
Option B: time in zones
This is closer to what many coaches mean by 80/20.
- Track minutes in low intensity vs high intensity.
- Ignore tiny spikes (watch noise) and look at the weekly total.
Step 3: how to do this with Apple Watch + Strava
Apple Watch workouts give you heart rate charts and time in zones for a single session.
To get a week-level view:
- Sync your Apple Watch workouts to Strava.
- In Strava, use the Progress views to see time in heart rate zones across your recent training.
- Once per week, do a 5-minute review:
- did easy days stay mostly low intensity?
- did hard sessions contain real high intensity?
- did you accidentally add a third hard day (group run, long hill commute, etc.)?
If you do not use Strava, any platform that summarizes time in zones can work. The key is the weekly review habit.
The most common failure mode: the moderate rut
Many runners fall into what some coaches call the "moderate rut":
- easy runs are too fast to recover from
- hard runs are not structured enough to deliver a clear stimulus
Signs you are there:
- every run feels like a grind
- pace stagnates
- your HR drifts up on easy runs
- you need more caffeine to feel normal
A quick fix is boring but effective:
- slow down easy runs for 2-3 weeks
- keep just 1-2 hard days
- keep the long run truly aerobic most weeks
A simple weekly checklist
Use this every Sunday:
- Did I do 1-2 hard sessions? If it is 3+, that is usually too many.
- Did I do at least 2 easy sessions that felt genuinely easy?
- Was my long run controlled? (Unless it was a planned workout.)
- Did my sleep or resting heart rate trend worsen? If yes, reduce intensity next week.
Video: why intensity distribution matters
Where Century fits
Intensity distribution is powerful, but it is incomplete if you ignore recovery.
Century helps you see whether your current training balance matches your physiology by connecting:
- time in zones
- weekly volume
- sleep and consistency
- HRV and resting heart rate trends
So instead of guessing, you can make small, confident adjustments like:
- keep the plan but add an extra easy day when recovery trends down
- move a workout 24 hours later when readiness is low
- increase volume only when recovery signals support it
Quick disclaimer
This article is for educational purposes and is not medical advice. If you have a medical condition or symptoms during exercise, talk to a qualified professional.