Building weekly mileage without getting hurt

Add gradually, hold every third or fourth week, and pay attention to sudden jumps in your longest run as well as to changes in your weekly total.

There is no formula for this, and the widely-quoted one is weaker than its popularity suggests. What does hold is a pattern: build for a couple of weeks, then hold for one.

What the evidence on the ten percent rule actually shows

"Increase your weekly mileage by no more than ten percent" is the most repeated advice in running, and the evidence behind it is thin. The arithmetic alone behaves oddly at the edges: ten percent of ten miles a week is a mile, while ten percent of sixty is six.

The largest study of it — an 18-month cohort of 5,205 runners published in the British Journal of Sports Medicine in 2025 — examined three ways of measuring a spike in running distance. Its finding, in its own terms:

• Injury rates rose when a single session exceeded the runner's longest run of the previous 30 days by more than 10%. Compared with runs that stayed within that, the adjusted hazard rate ratios were 1.64 for a spike of 10–30%, 1.52 for 30–100%, and 2.28 for more than 100%.
• For the acute:chronic workload ratio the authors report a negative dose-response relationship, and for the week-to-week ratio no relationship at all. Their own summary advises caution when relying on those weekly calculations, "as no association, or even inverse associations, between these approaches and injury risk was found".

What that does and does not establish

It does not replace the ten percent rule with a better ten percent rule. This is an observational cohort with self-reported injury, and the numbers do not behave like a clean threshold — a 10–30% spike carried a higher hazard ratio than a 30–100% one, which is not what a simple dose-response would look like. Nothing in it says that every individual run must stay within 10% of your previous longest.

What we take from it is broader, and it is our reading rather than the paper's instruction: pay attention to sudden jumps in your longest run, not just to changes in weekly mileage. The weekly total is the number most of us were taught to watch, and it may be the less informative of the two.

It is not useless — it will still stop you doubling your week — but it is a blunt instrument aimed at the wrong target. Adding ten percent every week indefinitely produces exactly the load curve it is supposed to prevent, and it says nothing at all about the single long run that is the better-evidenced risk.

The pattern that does hold

A common build-and-hold cycle

Week	Volume	What is happening
1	Baseline	Establish
2	A little more	Build
3	A little more again	Build
4	Back to around week one	Absorb — this is when the previous three pay out

The fourth week is not lost training. Adaptation happens during recovery, and a block without a lighter week accumulates fatigue that eventually reads as being unfit.

Change one thing at a time

Volume, intensity and frequency each carry their own cost. Raising two together is where most avoidable injuries come from — the classic being a new interval session added in the same week the long run got longer.

Add frequency first: an extra easy run is the cheapest way to add volume. Add length next. Add intensity last, and only once volume has been stable for a few weeks.

When to stop building

These are the signals worth acting on, and none of them are dramatic:

• A niggle in the same place that is still there after three or four days.
• Easy runs feeling harder at the same effort, for more than a week.
• Disrupted sleep in the nights after harder sessions.
• Resting heart rate drifting upward over a week or two.
• Losing interest in running you were enjoying a month ago.

One of these is worth watching. Two or more together is a signal to hold — or step back — rather than to push through. Anything sharp, localised, or worsening during a run is a different category: stop, and see a physiotherapist if it persists.

How long a base takes

Longer than the fitness. Breathing and perceived effort improve within weeks; tendon and bone adaptation runs on a scale of months. That gap is the whole reason building is slow on purpose, and it is why the runners who progress most reliably are usually the ones being slightly less ambitious than they feel capable of being.