Biking the Middlesex Fells
Back when I worked at Sybase in Burlington, I used to bike the Fells three times a week (at least!). I just kept the bike in the car. I've manned the booth there, too, to try to get people to bike more reasonably.
I've started from many different spots, tho my current favorite is by the small pond. We'll go straight, left around the pond, up the hill, and then loop in there. Often we'll go across the road as part of our ride and do that small "quiet" part as well.
"Mountain Biking Near Boston"
an excellent reference with maps!
Biking in New England