The primary device used to keep track of a marathon runner's pace is a stopwatch or a fitness tracker. These devices measure elapsed time and help runners gauge their speed and distance throughout the race. Many runners use GPS watches or smart devices that provide real-time data on pace, distance, and heart rate. Monitoring pace is essential for race strategy, enabling athletes to manage their energy and optimize performance over the marathon's 42.195-kilometer distance. Runners can adjust their speeds based on data insights during the race.