You will need to set up an account. Once you're in, you can enter information about the run, such as the city, and save it for later use. I believe that this will allow you to import the map to e-mail or blog as well.
Once you enter the information, you can narrow down your area. Click on your start point (or nearest intersection) and then click at each turn. The site will keep track of the distance until you click on the "start" again (most of us run in loops, right?). You can also add flags for water fountains and other stops you may need to make.
I have since confirmed the accuracy of this system versus my Nike+ kit, and they are both within about 0.1 miles different, which isn't much for a recreational runner like me. If you think you can use this, just give it a try!