I suspect someone more knowledgeable than me will be along eventually to give you more info, but I think there's a good chance you can find what you're looking for!
You just happened to pick an awesome time to come to Pittsburgh because our city's BikeFest is happening August 5-19, so there will be lots of rides and other bikey things happening every day. The calendar is still getting settled as different groups submit info about the events they are going to host, but the Google Calendar on this page shows some of the rides and events that will be happening. Some of the rides might be at too slow a pace for your preference, but I bet you'll be able to find something that suits you!
I've never rented from them myself, but the main rental place I know of is Golden Triangle Bike Rental. Other people may have some other ideas as well.