Throw in the ~1 hour delay of your arriving flight, and you're down to 1.5 hours. This is ORD, after all.

There's a decent health club facility in the Hilton that's connected to the airport via underground walkway. Not sure if you exercise, but it's a great way to boost the energy level after a presumably long int'l flight.

If you're Hilton Silver and above, it's only like $7.00 to use.