This past Sunday, the Star Tribune newspaper just published a full page of trails in Minnesota. These are all rail to trail with a few exceptions. You could make a loop about 300-400 miles from Brainard up to Ely then back down to Grand Rapids. Along the way you could hook into the Arrowhead trail and if you are there after June 11, the new Cayuna Country Trail (An IMBA epic). Jay Cook state park has some trails and there are others pepperaed along the way.
For more ideas
www.morcmtb.org is the local IMBA chapter with lts of very helpful folks.