Running 26.2 miles requires months of training. Monitoring your heart's capacity to cope with such an endurance test during training will show you your average heart rate. Remember that you are an individual and that your average heart rate may differ from overall averages due to age and other physiological factors.
The average heart rate for a marathoner is 160 beats per minute during a race. Most people, excluding the super-athletes, run a marathon at about 65 to 80 percent of their maximum heart rate. That gives an average heart rate of 117 to 144 during a marathon for a 40 year-old man. A 35 year-old woman will have an average heart rate between 122 and 152 bpm. The 160 bpm overall average takes into account the age range of marathon runners and the maximum heart rate range.