If you're on a road bike, you can get to Concord from Boston on the Minute Man Trail which winds through Lexington and Bedford.  If you don't have that much energy, start out at the Colonial Inn (town green) and do the loop to Carlisle and back - take a right past the colonial Inn on Monument Street and keep going till you hit Route 225 - take a left (and stop for Ice-cream at Kimball Farms) go through Carlisle centre and turn left down Lowell Road back to Concord

Running & Hiking
Park at Walden Pond (route 126) and walk around the pond (about a Mile).  There are miles of trails forking off of the main trail.

The Minute Man trail is great for running and walking, although you have to turn around and repeat your path.

Up Monument Street about a Mile after North Bridge on your left is a small parking area for Punkatasset pond (and woods). There are no signs - this is another network of miles of running/hiking/mountain bike trails.  You'll meet plenty of runners and dog walkers in the woods.