Hey, Chris - "soft" might get you "muddy", if you're okay with that. The best bet from where you are is, as people have mentioned, Schenley Park -- but the trails still have a bit of snow & ice on them and are in mid-melt. For 60 min (how fast?), you'll have to be creative with mixing up the trails, but it's quite possible to get 9mi in Schenley without too much repetition.
If you want flat, run down to the south side trail (it's the other side of the river from the jail trail -- better views and less traffic noise). A very reasonable run is to head down Panther Hollow from near Pitt, cross the Hot Metal bridge, and then hang a left onto that section of the south side trail. Out & back you can get about 10 miles on that from Pitt. The profile is down about 200 feet and then flat (and then back). The trail is dry and nice right now, and there's a bit of dirt on the side of it that you can run on for about half of the trail. If you look at the first few miles of this run:
http://connect.garmin.com/activity/68188685
You'lll see what I'm talking about. (Just don't make fun of my pace while running with my girlfriend for the first three miles. :-)
Squeak if you want more suggestions.