+1 bikewashington.org - Great site!

I lived in Silver Spring and Aspen Hill for many years. Montgomery County along the Beltway is not the most commuter friendly for bikes. Many of the major roads like Georgia Ave. and 29 are fast moving (45-50mph) with lanes too narrow to share. The Sligo Creek trail rides behind Silver Spring up to Wheaton. It is an old school trail and is narrow and is curvy in parts. The Silver Spring post office services a very, very wide area and so for help you'll do well to provide an intersection. Now I last lived in Aspen Hill above Silver Spring, and to commute into DC I rode part of the way and I used the bus part of the way. All the buses have bike racks here. My knowledge of backroads in Rockville is nil.

I've seen on a thread here that there are rides between Bethesda and Rockville that are comfortable for commuters but I have no firsthand info. I know there is a hard packed trail called the Georgetown Branch (an extention of the Capital Crescent trail) between downtown Bethesda and to close to downtown Silver Sping. Bethesda is quite expensive but you may be able commute to both Rockville and to downtown Silver Spring from there.

Peace