We checked into this with the Mayor's Office of Special Events and found out that the Marathon course does not go on the Lakefront Trail.
We do anticipate that traffic on the trail may be heavier because of people traveling to and from viewing points (although the trail itself is too far away from the actual course for great viewing). That's why we list it on our updates.
Here is a link to PDFs of the course and approximate closure times: http://www.chicagomarathon.com/CMS400Min/Chicago_Marathon/spectator...
Hopefully that will help Chainlinkers get around and plan their trips and recreation for the weekend!
Ethan, with Active Trans