In this project, we measured overconfidence in a large, heterogeneous sample making familiar, repeated choices in a natural environment that provided direct feedback. Specifically, in study 1, we elicited predictions of own finishing time among participants of the 2012 Warsaw Marathon. The participants' prediction errors were highly correlated with the change in pace over the course of the run: overly optimistic forecasters slowed down more in the second half. In study 2, we consequently took this slowdown as a proxy for overconfidence and used existing field data of one million participants in several large marathons for which split times are available (but own predictions are not). Both studies indicate that men as well as the youngest and oldest participants tend to be more confident. In study 2, we were able to investigate national and cultural dimensions. We found confirmation of previously reported findings of relative overconfidence in Asians. Additionally, we show some largely novel results, in particular that relatively conservative societies tend to be relatively overconfident. Copyright © 2016 John Wiley & Sons, Ltd.