Lexington & Concord by Car, Bus, Train & Bike
You can see both these historic towns in a day by public transportation: bus, bike and train.
MBTA Commuter Rail Fitchburg Line train at Concord Depot. Lexington is served by bus, not train.
Planning Your Day
Lexington is 15 miles (24 km) northwest of Boston; Concord is 22 miles (35 km) northwest. The distance between Concord and Lexington is 7 miles (11 km). Plan at least an hour each for your outbound and return trips, whether by car, bus or train; two hours each way if you're biking.
The sights in Lexinton are close together; you can visit most in about an hour (but more time is more fun). In Concord, there's more walking, as you can see by these walking tours on ConcordMa.info. Walden Pond is 2 miles (3.2 km) from the center of Concord.
There are more MBTA buses and trains on weekdays than on weekends; more trips inbound to Boston on weekday mornings, and more outbound from Boston on weekday evenings.
Car
The 22-mile (35-km) drive from Boston takes about an hour all the way to Concord, but you'll want to detour off MA Route 2 to Lexington along the way. Parking is easy in both towns, but traffic is heavy inbound/eastbound in the morning and outbound/westbound in the afternoon, with lighter traffic between 10am and 2pm.
Bus
Take the MBTA Red Line subway from Boston or Cambridge outbound to the Alewife terminus, then MBTA Bus 62 or 76 from Alewife to Lexington Green. The journey from Boston's Park Street station to Lexington Green takes about one hour. (MBTA buses don't serve Concord, but MBTA Commuter Rail trains do.)
Train
MBTA Commuter Rail trains on the Fitchburg Line travel between Boston's North Station and Concord in about 45 minutes. Trains depart about every hour—here's the timetable. Unfortunately, the trains serve only Concord, not Lexington. (For Thoreau fans: 3 minutes after departing Lincoln Station [2 minutes before arriving in Concord], the train passes along the edge of Walden Pond, easily visible from the right side of the train.)
If you're a biker, you can bike between Concord and Lexington, and take your bike on the train in Concord or the bus in Lexington:
Bike
The Minuteman Bikeway extends from MBTA Red Line Alewife Station in Cambridge all the way to Bedford MA, from which you can continue along the Reformatory Branch trail past Great Meadows National Wildlife Refuge to Concord (map), a distance of about 20 miles (32 km, about two hours).
Note that you can take your bike on a train back to Boston: an MBTA Commuter Rail Fitchburg Line train from Concord back to Boston from the Concord Depot (map). Here are the rules for taking bikes on Commuter Rail trains.
If you prefer, take your bike with you on the train to Concord, visit the town, then ride along the Reformatory Branch trail and the Minuteman Bikeway to Lexington, visit there, and do the return trip from Lexington along the Minuteman Bikeway to Cambridge and then to Boston. (Total travel time, train + biking, about three hours, plus rest and sightseeing time in both towns.)
You may also be able to take your bike on the bus (MBTA Bus 62 or 76 ) from Lexington to Alewife Station in Cambridge, limiting your riding to the trip between Concord and Lexington (less than an hour: map).