Which way are you going after leaving Assateague?
If North, I would suggest I-66 east to US17 south to Newport News Va, then taking I 64 east to meet with US13 so you can cross the Chesapeake Bay bridge Tunnel northbound from Norfolk.
If you are going south after Assateague, I would still take I-66 to 17 South past Fredericksburg to 301 North to US50 in Annapolis and across the bay there. And continue on 50 to Salisbury Md, then US13 south to Chincoteague/Assateague.