Sweeping vistas, rolling hills, and plenty of well-maintained trails to eat up.
Great spot for a run! Very accessible for those in the DC region.
A good starting point is the Henry Hill Visitor Center. If you want to avoid any crowds, start early. From the Henry Hill Visitor Center, you can't really go wrong (as far as direction is concerned) since most trails can be linked together to form a loop. I started on the First Manassas Trail (pictured) and it was phenomenal, starting on a bald hilltop and then moving into a wooded area. The trails are wide and extremely well-maintained, either gravel or dirt, but its hill-heavy and you'll find yourself doing a bit of a "roller coaster" along the way. Not much shade, so start early during the warmer months.
Dog friendly, too! Bring along your trail pup for some company.
*Distance is determined entirely by you, so if you want to make it longer, just string together more trails and you'll find plenty of room to roam.