The national MX track near Millville is also in the same general area as Kellog and Mazeppa. There's also a track at Elko. I don't MX so I can't tell you much more about them.
For trails, there's nothing public close to the cities. Closest would probably be Nemadji state forest south of Duluth. There are some trails south along the Mississippi on either side of the town of Weaver. Snake Creek and Trout Valley. Fun put not single track and they're multiuse so you could find horses, mt bikers, hunters, hikers, whatever.
Our club, the Golden Eagles out of Rochester, puts on two very fun trailrides each year at Theilman, MN which is near Plainview. See the armca site for details.