There are 5280 feet per mile X 26.2 miles = 138,336 feet. Since there are two sides to the streets X 2 again = 276,672 feet to a full marathon course.
Now, if we calculate the human body to be 2 feet wide at the shoulder, we divide the 276,672 feet along the marathon course by 2 feet to get the number of people standing shoulder to shoulder for 26.2 miles. That = 138,336 people standing one-deep, shoulder-to-shoulder along both sides of the route. To get one million people on a race course you would have to have thme stand seven deep the entire way, both sides of the street. Now, how many people do you think actually watch a marathon?