This old railroad trail (Northern Central Railway), also known as the Torrey Brown Trail, is located 15 miles north of Baltimore and is a good spot to rack up some miles! The southern end is in the Cockeysville suburb of Baltimore, off Ashland Rd. The mainly wooded, crushed limestone path heads north toward Pennsylvania. At the 21-mile mark is the border with Pennsylvania and the official Mason Dixon Line. The path continues as the Heritage Rail Trail Country Park (also known as the Ma & Pa Trail) for another 21 miles to York, PA. Flat, with mile markers along the way.
From the southern end in Cockeysville, a good out and back is the nearly 5 miles to Sparks Park (so, ~10 miles out and back).Full info on Trail