The Cat Creek area of Gifford Pinchot National Forest is currently the only area where roads are open to all vehicles. Even with your “street legal kit” and green restricted plate with ON-ROAD tabs, you are still not legal anywhere other than the roads marked for All Vehicles on the MVUM at this time. Those roads are what is covered in this book.
Most of these roads are fairly smooth. As you get further North and East away from the main NF-21 road and campgrounds, it gets rougher, but it never becomes challenging. Some of the roads are little-used by wider vehicles and are becoming somewhat overgrown.
If you stay on the designated routes, the entire area mileage is about 36 miles. But you will likely do some backtracking, so you could easily put on 75 miles in maybe 4-5 hours of riding.
Get details including directions, staging areas, maps, and difficulty levels in the UTV Guide to WA State.