I'm in central WA and most folks will run out of time and/or juice before there's an end of trails to beat around on. 30 miles unloaded is considered a big day out here though, and depending what you get tangled up in there are some 18-20 mile routes that take 7 or 8 hours even by pretty strong riders.
For starters, download the ORV maps
http://www.fs.usda.gov/recarea/okawen/recreation/climbing/recarea/?recid=57117&actid=37Anything open to motos is fair game, just double check those red lines as many of them don't allow bikes.
There's a lot more unmarked stuff out in the west fork Teanaway, but you kinda have to have someone who's familiar with the area get you started. I have a buddy who took off w/ friends from Roslyn up over the ridge into the 'way and wandered around in that drainage for ~3 days before they finally headed back.
There's also more trails and green dot roads out of Joe Watt/LT Murray just SE of Taneum, but given the size and terrain of Taneum + Manastash + Naches already... we'll maybe see ya in a week, lol.