Splendid Isolation 201K is now an official RUSA permanent! Best time to ride this is from about mid-May to early November. There might still be ice on the gravel roads otherwise.
Since this is a free-route permanent, you even can do a nearly all pavement version if you really wanted to…(although I am not sure I can recommend that).
Recommended Route: http://ridewithgps.com/routes/3504827