The Greenbrier Trail is a 78-mile, unpaved, multi-use trail running North-South along the Greenbrier River. "Mile Zero" is actually Mile Marker 3, at a parking area in North Caldwell. Keep this in mind when planning daily mileage. The northern terminus is more accessible from Northeast Ohio. It's a 5-6 hr drive to Cass, depending on whether you travel on I-77 or I-79. We chose to drive to Elkins and spent the night in a Hampton Inn before traveling the last hour to Cass the next AM. Dinner was at the nearby Fox Hops Brewery, which had just recently openned in spring 2024. There are some lodging options in the Cass Scenic Railroad State Park, and the nearby Snowshoe ski area that we didn't explore.
The trail runs "downhill" from Cass at subtle average grade of 1% at most. The trail surface is variable limestone/gravel, often doubletrack, and is somewhat similar to sections of the southern portions of the Towpath Trail (around Bolivar, OH) and Chesapeake and Ohio Canal. It's a reasonably smooth surface, with some rougher sections nearer the south end. Nothing scary.
There are a number of hiker/biker/boater sites along the trail. See the listing attached to the map (linked above). Not all sites have water or toilet facilities. Shelters are often provided and are very welcome during thunderstorms. These are good places to get out of bad weather while cooking or hanging out with friends at camp. There are also some alternative camping sites along the trail including a city park in Marlington, the River Trail Hollow Campground north of Seebert (no walk-ins, reservable only via Hipcamp.com), and the Riverside Campground at Watoga State Park (across the river and south of Seebert, but sites may all have a gravel base for RV parking).
If you are looking for food or resupply, your only options along the trail are Marlington (several restaurants, etc) and Jack Horner's Corner in Seebert. We ate twice in Marlington. Alfredo's had good food and large portions (closed on Mondays). The Greenbrier Grill and Lodge was slow and a bit chaotic on the day of our visit. The food was OK but nothing special.
There are shuttle services and B&B options for those interested in these services. We didn't use either.