It's fairly common in athletes. 13% of the marathon runners tested [0] had hyponatremia. This was at the Boston Marathon, a fairly exclusive race with experienced runners.
Runners with slower times, over 4 hours, seemed to be more likely to suffer. The announced qualifying times for the 2022 Boston Marathon include 3:55 for women 50 to 54 and 3:50 for men 60 to 64.
Running a marathon (or any endurance evening) is an extreme activity that is in the category of "make it happen". It burns double or more the calories of any pro sport.