Couple others:

Head out to Pike Lake, then to Esko, hook-up with Munger Trail (via gravel road), then back to Duluth.

There are some cool off road trails just outside of Jay Cooke on the Duluth side (need CX tires or a MTB), not sure on the conditions these days, but the last time I went, ATVs had more or less ruined what I used to consider the best MTB ride in the area... what used to be nice trail is now miles of ruts.

There is a new MTB trail opening up this spring off of skyline, not exactly sure of the location/details other than it is supposed to rock.

There are also a handful of trails inside of Jay Cooke, not sure on which ones allow bikes these days (used to ride before any were marked for just biking/hiking/horses etc.) Most could be ridden with a CX bike.

.... make sure the trails are not marked with blue paint, which denotes the Superior Hiking Trail. (unless the trail is specifically marked for both (not sure if there are any sections like this))