A positive split means your second-half time is slower than your first. In marathon running, significant positive splits — where the second half is 5–15% slower than the first — are associated with "hitting the wall": the severe slowdown that occurs when glycogen stores are depleted.
Positive splitting is the most common pacing error in marathon racing. Mass participation data consistently shows that the median recreational marathon runner runs the second half 6–10% slower than the first. The cause is usually going out too fast in the first 10K when energy is high and the effort feels easy.
The physiological mechanism: running at unsustainable pace depletes glycogen at an accelerated rate. Once glycogen is severely depleted (approximately 30–35 km in most recreational marathoners), the body shifts to fat metabolism alone. Fat metabolism is slower and less efficient; running pace drops sharply.
The Riegel formula, which this site's finish time predictor uses, does not directly model pacing — it predicts time based on maximal race effort at both distances. A runner who heavily positive-splits their marathon will record a worse time than their Riegel prediction from a half marathon implies.