Along with the Mid-Atlantic states all US States can be searched for any outdoor adventure. This site contains a good overview of all Maryland trails and outdoor activities. More of a state web page. Includes a list of several outdoor activities such as hiking, kayaking, etc. Trails and parks are given but only at the state park level not national.
Maps are available here too.
You can find distances and directions from your home to the trail head. This is a general Maryland site. Devoted to the outdoor activities in Maryland. Not just Backpacking, but fishing and canoeing.