I agree that it is a lot of walking. I have walked a similar route a few times -- Union Station to Soldier Field to watch the Packers beat the Bears, then to River North for dinner, then back to Union Station. My purpose was to try to string together everything on their list, which included the River Walk & Lakefront path. In reality I'd do more of what has been suggested here -- including the Art Institute and boat tour. Since they are limited to about 8 hours I'd suggest starting at Millennium since it's near the Orange Line, hitting the Art Institute as you mentioned (which probably means bailing on the museum campus in order to spend any meaningful time here), a 15 minute walk to the Architecture Boat Tour dock for the boat tour (http://www.architecture.org/tours/boat-tours) which will take a couple of hours, leaving some time to stroll north on Michigan Avenue and getting something to eat, then back to the Orange line. A lot less walking without trying to do too much, but still getting a solid feel for the city.

To the OP: Bring a jacket and gloves as the weather can range from the 40s to the high 70s that time of year, and will be cooler on the water. If the weather is bad, plan more time at the museums.