Lots of good trails at Pisgah for the MTB, but unless you are a diehard avid MTB guy, I would instead recommend Dupont State Forest. Good explanation and GPS maps of all the trails here:
http://www.mtbikewnc.com/maps/dupont-state-forest/gps-map
For road bike, if you don't want to do any real climbs, here is a nice ~60 mile loop through the valley (can be shortened or lengthened)
http://goo.gl/maps/lYi0Q If you want to take advantage of the topography, the climb up 276 and along the Parkway then down 215 and back is nice
http://goo.gl/maps/YP7lV or you can skip the Parkway and keep going down the other side on 276 until it reaches 215 then go up and down 215. If you take the Parkway, there is no place to refill bottles until you get back down to Balsam Grove, and the store there only takes cash. Also, there is a short tunnel at Devil's Courthouse on the Parkway, so legally you are supposed to have a head and taillight on for this section.
PM me if you want any details on places to refuel, bike shops, etc Tons of other good riding, just these are my quick recommendations.
"Some folks plan for the optimum race, others like to get upset when others out plan them. "