A couple good resources below. I hiked extensively in SNP when I lived in the DC area a couple years ago, but have since moved, though I doubt the info is out of date.
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(1) Here's my recommendation for a trail map of SNP
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(2) Hiking Upwardsis the best site for doing research on potential trails. Of course, you don't have to follow any one defined itinerary, but you'll get a sense of what's available and what spots are worth prioritizing.
Three ridges loop near Wintergreen is beautiful but only 15 miles-you could add on an out and back, even do The Priest which is one of the longest climbs on the entire trail. And there are tons of loops in Shenandoah that you can piece together. I have the Shenandoah map and just piece together trails to get the distance I want-pretty much infinite combinations without having to retrace your steps. http://www.amazon.com/Shenandoah-National-Geographic-Trails-Illustrated/dp/1566953650 http://www.hikingupward.com/GWNF/ThreeRidges/