Welcome to Boston and environs; I love riding in and around this town…
For some generalities, my favorite map is the AAA road map of metropolitan Boston. I think of the area in sectors radiating from downtown and surrounded by a circumferential belt about 10 to 15 miles from Downtown, known as Route 128 ("America's Technology Highway"). Unfortunately, 128 is a barrier to get through, especially on hair-raising roads that serve as feeders to the entrance ramps; over- and underpasses are more pleasant. All the riding is markedly better outside of 128, but the city and inner suburbs are nice and interesting. I'm an early mornng rider so I don't see the worst and my view may be through rose-colored glasses.
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Even though I've lived here for over 30 years, I always get lost on a new ride. Streets are laid out in a haphazard fashion; many streets, particularly the one you are riding on are not marked; they surreptiously change names; and in rotary intersections it's easy to lose your sense of direction. (I don't have a GPS.) On a happier note, the Transportation Authority (MBTA) allows bikes on subways and commuter trains with certain restrictions and that's a nice way to get out of town without city riding.
I would describe the sectors as (mostly for road riding outside of Rte 128):
North Shore: Beautiful Atlantic coastline, especially north of Lynn, to include Nahant, Marblehead and Marblehead Neck, on through Salem, Beverly and into ritzy Beverly Farms, and up to seafaring Gloucester, Rockport, Ipswich, etc.
Northern Suburbs: Lynnfield, Reading, Wilmington, Woburn, down through Winchester, etc: Pleasant suburban to rural inland roads.
Western: Lincoln, Lexington, Concord, Wayland, etc: Very ritzy, buccolic and historic; very popular for riding. This area IMO has the steepest hills.
Metrowest: Framingham, Natick; pleasant suburbs though pretty commercial along Rte 9
Southwest: Needham, Wellesley, Dover, Medfield, Walpole, Westwood, etc: probably more popular than the western burbs; wealthy exurban to rural, moderately hilly country roads, horse farms, mansions.
South; Norwood, Canton, Randolph, etc: middle class suburbia; rideable but usually on the way to somewhere else (no offense)
South Shore beyond Quincy and Weymouth and into Hingham, Scituate, Marshfield, etc: Atlantic coastal, nice riding, though I find it hardest to get to because of confusing suburbs and pretty heavy and industrial sections, especially Weymouth
[see EDIT, below] .
I am a solo rider but I think the Charles River Wheelman is the big cycling club around here. Some bike shops have organized rides, e.g. Landry's and Back Bay Bikes I know for sure. Wheelworks, International Bicyles and Harris Cylery are also well-known, but all are close to or in Boston. The Mass Bike Coalition, massbike.org is probaly also a good resource.
Feel free to PM me with other questions,and I look forward to comments from other area riders.
EDIT: This summer (2011) I have found a pleasant alternate route to the South Shore via Rte 53 and Broad St in Weymouth, and this sector is now a desirable area to ride…