The Cabin John trail runs roughly 10 miles North-South, from Cabin John on the Potomac River to Montrose Road & I-270 in Rockville. The southernmost 2 miles (from the Potomac to where it crosses under the Beltway - near Seven Locks Road) is not ridable and is not open to bikes.
Cedarville State Forest is a great place to experience twisty narrow singletrack without a lot of elevation gain or loss. It is a wonderful place to learn how to ride mountain bike singletrack. Each trail has its own flavor.