Long Mountain is on the Weyerhaeuser recreational permit area for Columbia - Washington Counties. It was previously activated via a 12 mile hike, but this is not the only route. As it turns out, there's a straightforward access trail via BLM land to the southeast of the peak, which only requires about 1.5 miles of hiking each way.
Driving route and hiking route GPX posted to sotamaps/SOTLAS.