From the Marathon Website:
Getting Around Town
Downtown closures will start at 11:30 PM on Saturday, May 3 and continue around the city through 2:30 PM on Sunday, May 4. For detailed information, view our road closure chart and road closure map.
If you need to travel around the city race day, view the Guide to Marathon Sunday map; routes in/out and around the city are provided. We also suggest checking our detailed map of Downtown before race day for anyone that will travel Downtown on race day; please note, certain sections of the city will be completely closed off to traffic on race day. Anyone in these sections of the city that will need to leave during the race, should consider parking their car outside the area.
– See more at: http://www.pittsburghmarathon.com/RaceWeekend#Town