Long-run development: the slow drift that builds the engine

The 10% rule — never increase weekly mileage by more than 10% week over week — is the most repeated piece of training advice in the sport. It's also not particularly well-supported. A 2018 review found only a handful of qualifying studies in this area, and the ones that existed showed similar injury rates across weekly increases anywhere from 10% to roughly 29%. Jumps of 30%-plus were where injury risk actually spiked.

So the rule isn't useless — it's just too rigid. A runner averaging 20 miles a week can probably jump to 25 without trouble. A runner already at 60 miles a week needs a more conservative approach because the absolute load is higher and the recovery margins are thinner.

What does hold up across the research:

• Step-back weeks matter more than the weekly percentage. Every third or fourth week, drop volume by 20-30%. This is what lets the work absorb. Without it, you're just stacking fatigue.
• The long run is the limiter. Your weekly long run should sit at roughly 20-30% of total weekly volume — beyond that, the long run starts to overshadow the rest of the week, and your easy days become recovery from one workout instead of building toward the next.
• Big jumps are riskier than fast accumulation. A 4-week block that adds 10-15% per week is safer than a single week that adds 25% all at once.
• Coming back from time off is its own pattern. Don't pick up where you left off. A common return curve is 50% of your prior peak in week one, then 25%, 20%, and 10% in the following weeks.

The shortest version: progressive but patient. The body adapts to consistent volume far better than to heroic single weeks, and the runners who get to the start line are the ones who treated the long run as a four-month project, not a four-week one.

Source: Luke Humphrey Running — Increasing Mileage: the 10% rule?