Okay, I live just south of MB. We have driven to WDW many times. It takes about 9 to 10 hours depending on the number of stops you make and traffic on I-95. We always do it in one day and it is no problem.
The fastest and easiest way to get to M.B. is to get of I-95 at Exit 33, Point South and take 17N to Charleston. It's a four lane road except for one small section between Gardens Corner and Jacksonboro. When you get to Charleston, take 526 around Charleston to Mount Pleasant and pick up 17N again. It's an easy, scenic drive to Georgetown. Take 17N through Georgetown and in Murrells Inlet, take 17 Bypass to MB. This is our favorite route because except for Charleston, the traffic is usually very light because most tourists don't use this route to MB.
We have tried taking 521 and 378 from I-95, but they aren't any faster and I hate I-95, to 17 is the way to go.
Enjoy your stay at "our" beach.