If you're looking for a meeting place or a good stop around ½ or halfway between Pittsburgh and Myrtle Beach, you can try Pulaski, VA, which is about 5 hours south of Pittsburgh, or about 5 hours northwest of Myrtle Beach.
Other possible places to stop near the midpoint include Mechanicsburg, Fort Chiswell, and Galena.
About ¼ or a fourth of the way, you can stop at McWhorter, WV. This is about 2 hours south of Pittsburgh.
About ⅓ or a third of the way, you can stop at Strouds, WV. This is about 3 hours south of Pittsburgh.
About ⅔ or two thirds of the way, you can stop at Greensboro, NC. This is almost 7 hours south of Pittsburgh, or a little over 3 hours north of Myrtle Beach.
About ¾ or three fourths of the way, you can stop at Thomasville, NC. This is about 7 hours south of Pittsburgh, or almost 3 hours northwest of Myrtle Beach.
If you drive straight through without stopping at all, the total distance is 612 miles. The distances shown above don't include local roads to get into each town, so if you're planning a long road trip, make sure you factor in traffic and road conditions, rest stop breaks, and interesting detours along the way. If you're in a rush, you can do the trip with zero stops, and your total travel time would be around 9 hours, 51 minutes.