Over all this is a decent app. Individually tracking 40,000+ is impressive.
The disappointment is that the claimed tracking isn't real-time tracking. It's an estimate with check-in points every 5 k.
That brings me to the second strange quirk. It's in Chicago (which uses Imperial measurements), it's advertised as a, "Marathon" (which is 26.2 imperial miles) but the check in points are metric... Like it or not, 90% of American users will not be able to visualize the metric distances and be able to tell where the competitors are on the track.
Finally, ChiMarathon insists of zooming back to my location on regular, 30 second intervals without prompting. As I'm scrolling around the map to see the runner or locations, I would just locate the desired information and ChiMarathon would zip back to my location. I'd have to go back on the scroll hunt for the information I desired.
Basic functionality is good, just some weird things going on in the minds of the developers.