Local Parks Close to Haymarket and Gainesville
Manassas National Battlefield Park
This park, while rich in history, also has some great woodland hiking. We spent much time here when our children were young. It's easy to get to and the hikes are not too strenuous. We have maps of trails at our office, but you can also pick up guides when you visit.
Sky Meadow State Park
http://www.dcr.virginia.gov/state_parks/
This park is a short distance away in Fauquier County. They have trails which have some elevation change so they are a bit more strenuous that a woodland hike, but many of the trail options are short and are good for getting an aerobic workout in a beautiful outdoor setting. Pets are welcome.
Bull Run Mountain Conservancy
This is a privately owned outdoor preserve on Bull Run Mountain, with administration by the Bull Run Mountain Conservancy. However, day hiking is allowed to anyone who signs a waiver. Hikes take the visitor to the top of Bull Run Mountain for a scenic overview of the area. The Conservancy offers memberships for people who would like to become more involved, and they offer special events such as naturalist guided hikes, tea parties in the woods, and nature camps for children
Bull Run Regional Park
http://www.nvrpa.org/parks/bullrun/index.php
I haven't been to this park in a while, but I do remember it as a great place. I have several patients who enjoy the trails there now. We took our children here when they were small, and I also camped there as a Cub Scout. It was actually the site of my first camping experience. Back then, it was considered really far out in the country. It's interesting to think of how our area has changed, but I appreciate that there are still great parks to visit despite the increase in development.