Well - since no one else is speaking up - I'll take a stab...
It's been a few years since we made that trip - but it seems like it took us 5.5 - 6 hours to get from HH to WDW. I have no idea the exact distance - but the route couldn't be more simple. From WDW, you're just going to take I-4 over to I-95, and head north 'til you get to the exit for Hilton Head. (I believe there's directions in our guide books and/or the member website.)
It's a very easy drive.