There there are several tools to find well worn existing trails, or create your own. Just remember trail riding is somewhat different than road bike riding in that when you go off road, you're often quickly out in the middle of no where. So be advised to look for existing trials, keeping an eye on the terrain and distances. As with anything, the more planning and research you do, the better prepared you'll be.
Use Backpacking Route Planing as a start. Then try REI's guides as a primer on what to take with you.
If you're feeling ready to go off on your own, the following resources will help you put together your own routes. Just play with the software, until you're familiar with the way it works. You'll need to set up accounts for most of the sites below, however this allows you to track and maintain your activities.
Map my Ride - - Create a route
Ride with GPS - - Develop, catalog and share your routes
Strava - - Develop training plans https://www.strava.com/
