Check out Cuyuna Recreation area up by Crosby/Ironton.
It's all red clay so don't wear stuff you care about especially when it's wet, because it stays in your clothes. But, it's really tacky so you grip well.
I was up there about 2 summers ago, it had opened not too long before then and from what I remember it was prime. Flowy, all levels of single track, and 25 miles of it. It's beautiful up there too.