As for paved trail riding - in the heart of Boston there's the Charles River Path. It goes all the way around the river. There's also a little offshoot that heads towards Watertown. If you venture over to Cambridge, you can get on the Minuteman Bikeway at Alewife T Station and ride the 13 Miles to the end of the trail into Bedford. Once in Bedford there are a few crushed rock trails that go for another 3 or 4 miles, and of course the street riding is a little better there too.

I suggest you pick up the Eastern Mass Bike Map - should be available at any bike shop.