I know we're a lot farther away over here, but there is an annual Tour des Lacs two day thing (from Spokane to Coeur d'Alene and back) that rides its longest portion on a generally flat multi-use trail. It's technically one city to the other and back the next day, and there are multiple routes from short to super long. The easy option really is just 40 miles of flat trail riding one way, and 40 miles back the other way... but it is scenic.

http://www.roundandround.com/TourDes...ourDesLacs.php

I know there are others out here, but I'm not really tuned into them very well. My dad wants to start riding more, though, so we might set our sights on one and ride it together.