Enjoyed a three day trip along Thirteen Mile, including a trip up to the top of Thirteen Mile Mountain for incomparable views of rolling hills, meadows, and nearby peaks.
Saw deer and a moose, and only encountered a single tick - no rattlesnakes. There were a few springs along the way, and the trail was easy to follow (though clearly lightly traveled). Arrived Sunday of Memorial Day weekend and only encountered one group on the trail.
The trail meanders through forests and grassy meadows at a constant but very gentle incline, offering more and more expansive views of the surrounding country. If you go, definitely make time for the short hike up to Thirteen Mile Mountain.
This part of the state has an entirely different plant community from the Cascades - lush, old-growth ponderosa pine forests (with Douglas-fir and larch), ninebark growing densely along the trail, as well as serviceberry, strawberry, arnica, and a huge variety of other shrubs and wildflowers.
There are no craggy, alpine peaks like in the Cascades, but the rolling hills, parklands, and solitude all make this a very special area.