I loved this trail and will be going back. Despite this National Park being almost in my backyard, I have never been here before and thoroughly enjoyed the experience.
One thing you should know is that the Prince William Forest Park has a lot of trails in it. Korie (below) is right - it's really a series of trails that make up the Pyrite Mine Trail Loop. The trails are well maintained and are a mix of flats and moderate hills. Trail markers are a combination of typical color markings on trees and at trail intersections there are [sometimes] easy to read indications as to which direction to go on. It's easy to get turned around and on the wrong trail so I would highly recommend stopping at the visitor center and getting one of the good trail maps they have.
The Pyrite Mine placards were very informative and reveal quite a bit of history about the area that I was completely ignorant of. It was fascinating to see a new forest grown over what had been an environmental blight. But the mine served its purpose and with help from mankind the forest has reclaimed the area.
The Visitor Center was great and there are some excellent displays of local wildlife, a relief map (which was very cool) and some supplies and t-shirts and such to buy. The Ranger we met there was super-informed, very friendly and really seemed to know what she was doing.
BTW - if you're interested in orienteering, there's an orienteering kit you can get at the Visitor Center and a log-book so you can record your tracks.
I would definitely recommend this and will be going back again. Great trails.