Tons of options. The classic loop would be to leave from Town Park (in the town of Fayetteville) and head onto the NPS trails using the Fayetteville Trail. Work your way over to the Arrowhead Trail system and do a few of those trails in there before dropping the gorge on Craigs Branch Trail (fireroad). Once at the bottom of Craig's Branch, you will want to go right onto Kaymoor Trail. Run for about 7 miles until you hit the road (you are now on the Cunard Access Rd.) If you turn around now and repeat what you've done (there are options to not repeat most of the trails once you reach the top of Arrowhead but you will need to look at a map or load up a file on your phone. Do you use ridewithgps, strava, or know how to import gpx files?) you're looking at around 20 miles roundtrip. There's also the option to drop even lower down Cunard Rd (paved 1.5 mile downhill) and then hit the gravel road on the right until it turns into the Southside Junction Trail. Another option is to climb up the Brooklyn MIne trail. Tons of options. About 100 miles of trails/double-track/dirt roads are accessible from Fayetteville with very little pavement. The entire NRG region would put that number in the 250 mile range. I hope this helps. Go into one of the bike shops and get a trail map when you are in Fayetteville.
Check out the book Hiking and Biking in the New River Gorge. It lists lots of connection possibilities for trails both within and outside of the NPS boundaries and includes maps/elevation etc. As Joe said, there are a lot of options.
I’ll put in a plug for the trails at Babcock State Park. Short drive from fayetteville (25 min). Well established trail system. Decent maps available at the park office. Also, they host a few races every year including a half marathon which gets you close to the low end of your desired range. Very scenic running.