Marathon running has experienced considerable growth in recent years, fueled both by an increase in participation rates and by a corresponding increase in the number marathons staged each year. Because marathons have a presumed economic benefit, there also has been growth in the number of marathon-related economic impact studies. However, these studies calculate incorrectly, or omit altogether, an important source of impact: the impact generated when local runners use their home-city marathon as a substitute for participating in an alternative marathon out-of-town. Given that this pattern of behavior is common among marathon runners and the fact that local runners constitute a significant percentage of race participants in most marathons, errors in the treatment of locally based impact is an important problem that must be addressed. This study focuses on the proper way to account for locally sourced impact in marathon analysis, and presents a study of the 2006 Cincinnati Flying Pig Marathon to illustrate this methodology.