Easy! Go to this great section of the nytimes.com where you can see a time-lapsed video of the entire course traversed in just over 3 1/2 minutes. I ran it 10 years ago, but I still recognize the course after all that time and despite that speed. The main difference is that instead of cars, you will be surrounded by lots of runners and a fantastic NYC crowd that covers the entire course.
Despite it being short, you have a sense of just how long 26.2 miles is as you zip along.
There’s also a good interactive map that traces the run through the five boroughs, from all the way down in Staten Island to all the way up to the Bronx, before finally landing in Central Park.