I'm a new Landrunner (joined at the OKC Memorial Expo), thus my first posting.
I'm a big fan of the Sutton Wilderness Trail on the east side. It's on 12th Street between Robinson and Rock Creek Road (closer to Rock Creek- north of the dog park). It sits on the west side of the road on 12th.
It's a 1.5 mile loop, but it weaves through the trees and around a large pond that you can do 6-10 miles before you start to feel like you've seen this all before. It has a few side escapes that will take you to the ball fields on Robinson, and the like. The trail splits on the south side of the pond - thus if you stay close to the pond on the south side - you will link back up to the main trail, but you will only get in 1.25 miles.
My favorite place to run in Norman during the Summer (the shade in the trees breaks the heat a bit).