Seneca Creek State Park is 36.8 miles from my starting point. From the website:

“The park, comprised of 6,300 acres, extends along 14 scenic miles of Seneca Creek, as it winds its way to the Potomac River. The Clopper Day-Use Area contains many scenic areas, including the 90-acre Clopper Lake, surrounded by forests and fields. Picnicking, boat rentals, trails and a tire playground are just some of its recreational opportunities. A restored 19th century cabin and a self-guided path interpret the history of the area. Nearby, the Schaeffer Farm Trail Area offers 12 miles of marked trails for hiking and mountain biking. For the more adventuresome hiker, the 16.5-mile Seneca Creek Greenway Trail follows the entire course of the river. During May, the Schwartz peony gardens and field plants are in bloom. “