For those staying downtown there are several paved rail-trails which are part of the Three Rivers Heritage Trail system. They make very make nice runs for people unfamiliar with Pittsburgh's roads.
The North Shore Trail runs up the river from Heinz Field
past PNC Park towards Washington's Landing. As the trail passes
the 31st Street Bridge it is not quite as well maintained but still
very runnable up past the skate park. After it passes Etna area, it
turns into a maintenance road for the railroad. Travel this at your
Heading downstream from Heinz Field there is construction going on for the casino. This forces a poorly marked detour for a quarter mile or more. Beyond the detour is a nice trail that goes out toward the old penitentiary.
The Eliza Furnace Trail (a.k.a. "The Jail Trail") starts near the PNC building on Grant and 1st Avenue and runs east along the Parkway East for about four miles. It's noisy and smelly during rush hour, but on the weekends it is a nice run once you get out of the downtown area. There is also a lot of graffiti to look at along the trail, if you like that kind of artwork.
The South Side Trail is longer and quieter than it's cousin across the Monongahela river. From Station Square the trail heads up the Mon for several miles. About 3 miles out the new Hot Metal pedestrian bridge links to the Eliza Furnace for a nice 6-ish mile loop. Or continue past Hot Metal street for another couple of miles until the trail dead ends. It's an out-and-back run but very quiet and paved the entire way.
More information about these trails can be found at the Friends of the RiverFront web site.
The Montour Trail is part of the Allegheny Passage which runs from the Pittsburgh airport to Cumberland Maryland. It is not paved but during the season it has water stops and portalets at several points. You can find some good maps and information at their website.
If you have a suggestion for a run and we'll see about adding it. Include the web page, time, location, prices and beneficiary.