I haven't done much riding in the northern part of the state but folks rave about Elizabeth furnace and other trails in the Front Royal area but most are known for being extremely technical. There are some easier trails in Shenandoah st park just south of front Royal. Just a little farther down 81 you get to Bryce resort bike park which is known for flowy stuff that can be enjoyed on XC bikes.
A little farther still & you hit Harrisonburg. 20ish min southwest is Stokesville there you have it all. Literally hundreds of miles of trails from flowy (narrow back MTN & Tillman west trail) to extremely chunky but very rideable (Lookout mountain). There are far too many to list & most/all are xc bike appropriate. Just a little south from stokesville is the southern traverse which is a big ride, but not to be missed.
East of there (across I-81 via I-64) is Sherando lake/campground which has access to more great riding.
Southwest off I-64 I'd Douthat St park. There's at least 3 days worth of rides in the park with a few more just outside. Douthat also has a beautiful mountain lake with trout in it.
Farther south on I-81 is Roanoke there you'll find Carvins cove park which a veritable mtb playground with all types of trails from super flowy to super tech (over 40 miles of trail) with great maps & signage. There's lots of stuff between what I've listed to keep you busy. If you burn through all that with time to spare, from Roanoke, you're only 3-4 hours from Pisgah national Forest...