Progressive overload, and the 10% ceiling
Fitness comes from asking a little more of your body than last time, then letting it adapt. The trick is a little, not a lot, and the well-known 10% rule is best treated as a limit rather than a plan.
Ask for a little more, then adapt
Training works by repeated, manageable stress. You do slightly more than your body is used to, it repairs and rebuilds a bit stronger, and next time that same effort is easier. Push too lightly and nothing changes; push too hard and you dig a hole you spend weeks climbing out of. The whole craft is staying in the narrow band between.
Where the 10% comes from
A common coaching heuristic is to raise your long session or weekly volume by no more than about 10% a week. It is a sensible guardrail, but the evidence for that exact figure is mixed, so treat it as a ceiling you rarely need to reach, not a number you must hit. Some weeks you step up less. Some weeks you hold, or step back.
A ceiling, not a promise. TrailFree caps each working week at 10% over the last, then usually grows by less. Steady beats spiky.
Recovery is part of the load
Overload only becomes fitness if you let it settle. That is why sensible plans build in easy weeks rather than climbing forever. More on that in the recovery week.
Curious what a capped build looks like for your goal? Try the plan calculator.