The national highway system is great for rapidly moving from place to place. The problem, though, is that highways are congested and dangerous. One really cannot get the feel for the land from a road.
Fortunately, there has been a great deal of effort to build systems of trails that allow hikers and cyclists to experience the outdoors.
This section of CommunityColor lists trail related web sites.
National Trails System (www.nps.gov) The National Park System offers introductory information on designated National Trails.
County High Pointers Association (www.cohp.org) This project invites people to visit each of the county highpoints in their state. Some states are more difficult than others.
Continental Divide Trail Coalition (www.continentaldividetrail.org) Site has information on hiking the Continental Divide Trail in the Western United States.
Homemade Wanderlust (homemadewanderlust.com) This is a site by a thru-hiker.