Ride to Turtle Creek via Penn Ave and rte 130 from Blackridge.
Take 130 all the way to harrison city and then rte 993 out past Bushy Run Battlefield.
At that point, your on your own and unfortunately you are cutting across a relatively glacially heaved part of the world...lots of ups and downs.
that being said, I 130 is relatively flat and (to my recollection) 993 runs the valley until you get to about rte 66.
another option is frankstown road to rte 286 to (south) on rte 981 (pretty hilly, but not a super high traffic route).