TrailLink is an awesome site! It has trails from every state! And of course for walking but also all kinds of other activities, like below:
Find Trails by Activity
Cross Country Skiing
All routes on TrailLink.com are walking trails. Here you’ll find everything from short, medium, and long routes and from easy, moderate, and sometimes more challenging trails that include hills. Also included are some fitness trails purpose-built to engage communities in staying healthy. Walking paths listed on TrailLink.com pass through a variety of landscapes, including nature trails through city and county parks; urban passages that provide safe access across town; scenic trails with historical significance; and remotely located walking trails through forests and open rural stretches, or among lakes and mountainous regions.
When choosing the right walking path for you, consider the trail’s surface, length, and distance between amenities, such as water fountains, bathrooms, shelter, or food. On those longer trail walks, going with a friend is a nice way to share the experience.
For more info and to find your trails, go to: http://www.traillink.com/