<human>:  Why is it that the Boston Marathon is so hard to run?
<bot>: The Boston Marathon is a hard race to run because it is a long race. The Boston Marathon is 26.2 miles long, which is almost twice as long as the typical marathon distance of 20 miles. The Boston Marathon is also a hilly course. The average grade of the course is 2.5%, which is much steeper than the typical marathon course, which has a grade of 1%. The steepness of the course means that runners have to work harder to keep their pace up, and this makes the Boston Marathon much more difficult to run.