The route I've been taking is this fellow.
http://www.bikely.com/maps/bike-path/206594
The interchange in McKeesport is disgusting, and there are some pretty big hills, but it's doable. If you're laden, I hope you have a triple. The E. Pittsburgh-McKeesport Boulevard is build like a freeway in places, but barely trafficked at all. Another warning— a lot of the Mon Valley towns are pretty depressed, so you'd probably want to pass through them before dark.