A wind turbine rotor blade assembly and associated method include a rotor blade having a pressure side, a suction side, a leading edge, and a trailing edge extending in a generally span-wise direction between a tip and a root. An edge extension panel is attached along either of the leading edge or trailing edge (or along both edges) in a generally span-wise direction from adjacent the root towards the tip. The edge extension panels include a cured and hardened viscous material continuous core formed onto the leading or trailing edge with a contoured generally aerodynamic outer surface.