<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. It is a long race because the course is almost exactly 26 miles long. The course goes through the city of Boston, and there are a lot of hills, so the course is quite hilly. The Boston Marathon is also a very competitive race, with many people running the race each year.