It is really easy to leave Calais on small roads. On leaving the ferry and adter the French passport controls you will see a sign showing a slip road to the right marked "Centre Ville"
That brings you to a roundabout. Straight across and after a short while (about third of a mile) turn left for various small towns (le Fort Vert, Walsam) which takes youi on a humpback bridge over the motorway. Then Gravelines, Grande Synthe, Dunkirk, Zuydcoote, Bray Dunes, Belgian coastal route to Knokke (largely on sea front and dedicated paths), Breskens and ferry to Vlissingen, then any number of routes through Holland and along the Rhine.