I don't know of a one-way trail or a loop that long off the top of my head right now, but you could string together several different trails together in the Cohutta's to make a solid trip that distance.
Here's a pretty good link that will give you a trail listing:
http://www.greatgeorgiaproperties.com/Outdoor-adventures/Hiking-trails-of-the-Cohutta.htm
There is also a good guide book from Tim Homan that will give more than enough info you need.
Cohutta advice: watch the weather forecast b/c there are tons of river crossings on the Conasauga and Jacks river.