Nice, another PH resident, welcome to the board
Depends what part of Penn Hills you are in. I'm in Rosedale, which is just north-east of Crescent Hills, very close to ARB. I've seen a bunch of people bike towards town on ARB in the mornings (during bike count I think around 6 or 7). When I head downtown, if I'm in a big hurry, I go down ARB->Butler->Penn->downtown. Lawrenceville can invovle a sneak over to some clearer streets, but they're narrow and full of stop signs.
I've also crossed the Allegheny at the 40th and taken the trail, but wasn't in a rush. Just prettier. Crossing up in Oakmont puts Cabbage Hill (Etna) in the way, though Freeport road is easy and you could take the train tracks to the trail, all flat. Not so sure about it on a regular basis, though.
If you're not headed all the way to town, Saltzburg->Verona->Frankstown works nicely, then there's any manner of ways to get closer to down town from East End.
If you're from closer to 380, just cruise Frankstown all the way to East End, then any manner of way to get closer in.
If you're starting closer to Chalfont or Churchill, I have no idea. I'd try Old Wm Penn to Penn, but Penn gets squirrely. There's others on here who can talk about the southerly routes better. Maybe through Braddock and taking advantage of some of those trails down there.
Best of luck!! (and let me know what you figure out, I'm always looking for better routes)