If you want to get in some miles on a flat off-road trail, the William Munger State Trail is a great option. This is actually a collection of several multi-use trails between Duluth and Hinckley. The closest starting point to Duluth is the trailhead about 6 miles southwest of downtown, near the Lake Superior Zoo. From here, the trail heads southwest for 6 miles to Short Line Park, and then for another 7 miles to Carlton, passing through Jay Cooke State Park. There are numerous other spurs — see the attached trail information and trail map.
The trail is one of the better options for fairly flat running in the area. It passes through woodlands, and some giant rock formations — especially the sections in the State Park.