I have driven from Williamport and on to PA 14 at Trout Creek. You will steadily gain altitude as you head north from Williamport. PA 14 wasn't in the best shape the last time I was up there about 4 years ago. What shoulder there was was deteriorated and/or covered with gravel in a lot of places. However, traffic was light.
Outside of Williamport, there is a something called the Pine Creek Trail:http://www.visittiogapa.com/railtrail.html
It takes you through the "Pennsylvania Grand Canyon." It would take you west of Route J, but I believe at the trail terminus in Ansonia it's relatively easy to get back onto Rte. J. Just take U.S. which is part PA Bike Route Y in that area, east to intersect with Route J. One problem up in that part of the word is the proliferation of truck traffic associated with gas drilling.
I have also driven U.S. 11/15 north from Harrisburg. Expect heavy traffic in places when you are on it, especially in the Selinsgrove/Shamokin Dam area.
Furhter south, U.S. 322/22 is a divided highway. Use Google Street View for that road around Duncannon and see what I mean. Again, I have driven that road. I would never want to bike it. Certainly avoid it during rush hour. It's a main route into Harrisburg.
From where you cross the river at Duncannon, PA 147 on the east side of the river looks infinitely nicer and less travelled. You could cross back over to Shamokin Dam at Sunburry.