"To break 4 hours in the marathon, you need 200 km a month." "Break 2 in the half? At least 150." Once your race entry was in, you probably searched for a formula like this — and then felt a small wave of anxiety looking at last month's 90 km. Here's the short version: these formulas aren't wrong. But they answer "how much does everyone run on average," not "how much should you run." Those are two very different questions.

Key takeaways

  • Formulas like "sub-4 = 200 km a month" are statistical averages — a portrait of a crowd of finishers, not a prescription written for you.
  • The same monthly mileage can be steady accumulation for runner A and the edge of injury for runner B. The difference isn't the number itself — it's your load history: how much your body has recently been used to.
  • There's a simple tool for reading that history: ACWR. In plain terms, it's "your last week's volume" divided by "your average weekly volume over the last four weeks." Between 0.8 and 1.3, injury risk is at its lowest; push past 1.5 and the risk climbs sharply.
  • So a better question than "how much monthly mileage is enough" is: "given what I've been running lately, how much can I safely add next?" The usual answer: at most about 10% per week.

The formula isn't wrong — it just can't answer for you

Where does "sub-4 = 150–200 km a month" come from? Spread out the training volume of a large group of sub-4 finishers, take the average, and you land somewhere in that band. As a distant landmark on a map, it has real value: it tells you roughly which direction the destination lies.

The problem starts when you treat the landmark as a navigator. Averages erase the single most important piece of information: everyone starts from a different place.

  • For someone whose monthly volume has been stable at 180 km, a 150 km month is an easy month.
  • For someone who ran only 60 km last month, forcing 150 km this month means pulling training volume up 2.5x in one go — the most classic injury script there is.

Same 150. For one runner it's accumulation; for the other it's a gamble. The formula can't see the difference, because the formula doesn't know you.

Where's the difference? In your load history

Your body has a "habituation level": however much load you've asked it to carry over the past few weeks is what it's prepared to carry. Injury risk doesn't follow "how many kilometers you ran this month" — it follows "how far this volume exceeds what you're used to."

Sports science has a simple tool for exactly this, called ACWR (the acute:chronic workload ratio). The name is stiff; the idea is plain:

What you ran in the last week ÷ your average weekly volume over the last four weeks.
  • Ratio between 0.8 and 1.3: you're adding volume close to the range your body is used to. Lowest risk.
  • Ratio of 1.5 or above: you're ramping up much faster than your body has adapted to, and injury risk climbs sharply. A study tracking 735 New York City Marathon runners found that injured runners had spent more weeks in this zone.
  • Ratio persistently too low (below 0.5): your body is detraining, and fitness slowly drains away.

(In practice, newer methods weight the most recent days more heavily, so a single sick week doesn't distort the picture — but the concept is exactly this ratio.)

Back to the earlier example: a 60 km-a-month runner charging to 150 km jumps from roughly 14 km a week to 35. The ratio blows straight off the chart. Their problem isn't that "150 isn't enough" — it's that "150 is too much for who they are right now."

So the better question is: "what's my next step?"

Swap "how much monthly mileage is enough" for "given what I've been running lately, what's my next step" and the answer becomes something you can act on:

  1. Start with an honest audit of your last four weeks of real mileage — from the records on your watch, not from the runner in your memory.
  2. Cap weekly increases at about 10%. If you run 30 km a week, 33 km next week is a solid step; jumping straight to 40 is betting against the ratio.
  3. Work backward from your goal to budget the time. Going from 90 km a month to 180 takes roughly two to three months at a safe rate — which is why mileage is something you start saving the day you register, not something you cram in the month before the race.

The numbers this produces may look more conservative than the formula's. But they come with something the formula can never offer: this is a volume your body can actually absorb.

Your mileage ledger, kept by your data

You can run all of this on a calculator yourself. But tracking your own four-week average, watching the ratio, and working out the next step every single week is, honestly, a chore — and that's one of the reasons we built Paceriz. It reads your real records from Garmin, Strava, and Apple Health, keeps this mileage ledger for you, and tells you directly each week: "here's your next step" — and why. Because last week's volume jumped too fast, or because you actually still have room to add.

Not "what everyone runs on average," but "what your next step is." Now you know which of the two questions to ask.

One last thing

The formula is other people's average; your body only recognizes its own history. Whether your monthly mileage is enough isn't something you measure against someone else's answer. Start from your real mileage over the last four weeks and build upward one step at a time — slower, perhaps, but every step lands on solid ground.

Want to know what your next step is?

Let Paceriz read your real training records and keep your mileage ledger for you.

Start your first week free