The Chicago Lakefront Trail has off the charts usage. The marked multi-use, paved path received significant damage yesterday due to high winds, however two friends and I ran it point-to-point-to-point today using indoor bathroom facilities once and Chicago Park District maintained water fountains. The Trail in one direction is marked every half-mile and stretches along 18 miles of the Lake Michigan shoreline.
I've wanted to do this run since competing in the Lakefront 50k in 2011 but kept putting it off. Unfortunately, I picked a tough day for the attempt as the weather (72 > 86deg, 66 dew point, sunny) turned out to be way worse than originally forecast. I ran unassisted using water fountains to hydrate along the way while consuming five gels. I did have company for the first twelve miles of the run, which helped take my mind off the task at hand early on. Even though my running pace stayed relatively consistent throughout (avg. 6:44m/m), I lost a lot of time in the last nine miles as my water stops became longer and longer; however, this was necessary to keep from overheating.
While the Lakefront Trail isn't a particularly scenic venue for an FKT, it has a certain appeal given it's heavy usage (as Dan alluded to above), especially if you live in the area and use it regularly. The heavy usage is also a nuance, though, since there tends to be heavy foot & bike traffic from the aquarium up to Montrose Beach. Additionally, the flat monotony of the path along with its cambered concrete/asphalt and the limited shade take a toll on the legs and mind.
** The path is marked out to be eighteen miles, but it appears that the final marker on the south end is inaccurate, making the total distance a bit short of 36 miles. My best guess is that when the city repositioned the markers a few years ago they weren't able to place the southern marker any farther down the road (since it's a dead end), so they placed it as far south as they could, which turned out to be short.