Rt. 128/95 to Exit 29, Rt. 2 West. Go 4.6 mi., look for the Rt. 126 sign, and turn right at that intersection onto Walden Street. Bear right at the fork. At 1.2 mi. in Concord Center make a left onto Main St. and a right into the town parking lot. The Visitor Center is at the south end of the parking lot. By commuter rail and bicycle: From the Concord station, head north on Sudbury Road, which merges into Main Street, then take the first left onto Keyes Rd -- about 1 mi. Train schedule is here: https://mbta.com/schedules/CR-Fitchburg/timetable
18 miles around Concord Center, some road but mostly on dirt. Doable in 2 hours but, if time runs short, the last loop can be eliminated saving 45 minutes.
Bring your gravel or mountain bikes. Non techy and a good intro to riding in the local woods if you haven't done it before.
Download the latest, revised route. More dirt, less road, and a bit less distance.