Chicago is one of the best marathons in the world. Starting the race in downtown, circling back a few more times, and finishing back near Lake Michigan, this is a cool experience you cannot get anywhere else. There are tons and tons of people, so make sure to get your corral qualifier entered to move near the front. The back half of the course is pretty exposed and can be hot if the weather is sunny, but really cool race. Great expo too, a must do at least once.