In Short
Middlesex Fells' 2,000 acres of pastures and woods, reservoirs and rocky hills are just minutes north of Boston. Trails range from wide and gravelly to narrow and twisted, and Interstate 93 cuts the park in half. The longest loops and most of the parking lie west; the eastern trails are more technical, but dump onto city streets around Stone Zoo. Bikes are banned on most single tracks, including the hiker-only Skyline Trail, so mind the signs or face a hefty fine. The New England Mountain Biking Association and the MDC have, however, mapped out a six-mile loop that includes stretches of single track on which bikes are allowed and some tricky switchbacks.