The familiar 10% weekly mileage rule is not a proven boundary between safe and unsafe training. Recent cohort evidence suggests that large jumps in the distance of a single run may matter more than simple week-to-week mileage ratios. Progression should therefore be judged in context, not by one universal percentage.
The 10% rule is memorable. That does not make it a law of running physiology.
A runner can increase weekly mileage by only 10% and still make one individual run dramatically longer than anything completed recently. The weekly total can look conservative while the session itself is a major spike.
The problem is that the familiar 10% rule is not a scientifically established line between safe and unsafe training.
A weekly percentage can hide a lot
Suppose you normally run about 40 kilometres a week and your longest run is around 12 kilometres.
The following week you run 44 kilometres in total. On paper, that is only a 10% increase. But if your long run has suddenly increased from 12 to 20 kilometres, one part of your training has changed much more than the weekly total suggests.
Recent research supports paying attention to that difference. A 2025 study of more than 5,000 runners found that large increases in the distance of a single run were associated with a higher risk of overuse injury, while simple week-to-week mileage changes did not show the same relationship.
That does not give us another “safe” percentage to follow. It tells us something more useful:
“Too much, too soon” cannot always be seen in your weekly mileage.
You should not have to calculate the answer yourself
This is where a training system like SmartRunning can do something useful.
You should not need to sit down every Sunday with a calculator and work out whether 8%, 10% or 12% is acceptable. Nor should one general rule determine progression for runners with very different recent training histories.
SmartRunning is designed to manage progression within its training logic, using the training you have completed rather than asking you to identify a supposedly safe percentage yourself. Its current controlled product principles explicitly include progression, training-load and recovery guardrails.
That does not mean progress should always be conservative. Marathon training must develop if you are going to become capable of running a marathon.
It means the increase should have a reason and fit the training around it.
Injury awareness is partly about restraint
There is a natural pressure in marathon preparation to keep moving forward. Race day is getting closer. The long run needs to grow. A good week can make you feel capable of more.
Sometimes more is exactly what should happen.
But progress is not improved simply by making every increase as large or as fast as possible.
An injury-aware plan should be willing to progress when the training results support it and equally willing not to accelerate simply because there are fewer weeks left on the calendar.
That is the useful meaning of avoiding “too much, too soon”.
Making the next increase fit the training that came before it.
Let SmartRunning manage progression in context rather than relying on a single weekly percentage rule.