Going from Myrtle Beach to HF there are no Tolls unless you come to DC-695-Dulles Toll RD. I'd make a correction to Mike in that I'd go from Opal where 17-29-15 meet to Leesburg on 15, 7 west about 3 mi and West on Rt 9 to a right on Harpers Ferry Rd (RT 671) which is the 1st Rd West of Hillsboro. There is a really nice Organic Restaurant on the left, 1 mi N on Harpers Ferry Rd called Grandale Farms. There is room to turn around up in back, food is mostly grown there in the spring-summer and they have a wine tasting room. You can also go straight thru Leesburg to 340 West to Harpers Ferry.
I 73-74 isn't finished and I81 takes you a long way west before you turn NW