I have ridden that route numerous times on the Great Allegheny Passage. Typically, I'll start at Boston and ride to Confluence in 8 hours. To start in Pittsburgh would add a few hours and to not go all the way to Confluence would save an hour so, all told, Pittsburgh to Ohiopyle; 9 hours or so.
The toughest part will be the Pittsburgh to McKeesport section. While there are some trail sections complete over that 9 miles, they are not connected and the interruptions do not allow easy access by road. I recommend riding the Eliza Furnace Trail and then taking 2nd avenue to the Glenwood Bridge and then the road through Homestead, Duquesne and into McKeesport.
Once you're on the Passage, it's smooth and level the rest of the way.