"LBS"s (had to look that up), there is a fantastic bike shop called Bikes Not Bombs, and another really good one called Ferris Wheels.
There's a bike trail from Forest Hills to Back Bay, and a bike trail from Jamaica Pond to Brookline with a slight gap to another bike trail to the Fenway area.
A little further from JP, but close to some points in the above, there is a bike trail along the Charles river from central Boston to Waltham/Newton/Norumbega area. There's another bike trail from Somerville to Concord/Bedford (not far from Walden Pond).
And there are other trails too, as well as many good streets to ride on. I have found that pretty much all of the streets in the metro and extended metro area are ridable, with a few exceptions here and there.
A suggestion for longer mileage rides would be to head out Perkins St., behind the Jamaica Pond and follow the BNB, bikes not bombs, route out to Natick through Brookline, Newton, and Needham. I'm not sure how well it is still marked, but there are maps of the area available at most bike shops. Once you get out past Brookline there are usually a lot of cyclist riding out of the Cutler Park area. In general, there are a lot of good long, quieter streets heading out of the city. It is easy to get lost, so I do recommend a map if you don't know the area, just in case. I usually end up riding around until I get somewhere recognizable.