The B&A actually runs all the way to just north of Annapolis; no need to get on Route 2. Go *all* the way to the end, then make a left onto the road, down the hill, make a right at the "T" onto 450, downhill some more, and shortly you will see the Severn River Bridge and the Naval Academy.

I'm hoping somebody pipes up with a good route to the trail from the Baltimore end, because I'm not sure about that. Did you know that the B&A Trail is linked at its northern end to the BWI Trail, a loop that runs around the airport? That would be closer to downtown Baltimore. 648 may still be the most direct route, but there may be better options in terms of traffic, road conditions, etc.

http://www.bikewashington.org/routes/bwi/index.htm

Maybe the new(ish) Gwynns Falls Trail would be an option, if you could get yourself to it? I've never been on it, but apparently it ends on Hanover Street in South Baltimore. From there you could head through Brooklyn and Curtis Bay, on Pennington and Patapsco Avenues, eventually becoming Fort Smallwood Road, right on Marley Station Road, cross Ritchie Highway into the Marley Station Mall parking lot, which (at the J.C. Penney) sits right next to the B&A Trail. The Brooklyn/Curtis Bay leg is kind of nasty, though -- lots of potholes and debris, heavy industrial traffic (not so much early on weekends, though), somewhat marginal neighborhoods.

http://www.gwynnsfallstrail.org/