Sunderland is the end/start point for two major cross-country cycle trails. The C2C (Coast to Coast) route and W2W (Wanley to Wear) are sign-posted as they cross the country from the West Coast to the mouth of the Wear. There is a link to the web site for these routes here: C2C and W2W cross-country routes
Mapmyride’s web site contains several ideas for shorter cycle routes in and around Sunderland. You can access these from their website here: Cycling around Sunderland