If you're looking for a meeting place or a good stop around ½ or halfway between Ashburn and Midland, VA, you can try Manassas Park, VA, which is about half an hour south of Ashburn, or about half an hour northeast of Midland, VA.
Other possible places to stop near the midpoint include Sudley, and Wellington.
About ¼ or a fourth of the way, you can stop at Chantilly, VA. This is about 13 minutes south of Ashburn.
About ⅓ or a third of the way, you can stop at Centreville, VA. This is about 18 minutes south of Ashburn.
About ⅔ or two thirds of the way, you can stop at Rixlew, VA. This is about half an hour south of Ashburn, or about half an hour northeast of Midland, VA.
About ¾ or three fourths of the way, you can stop at Colvin Farms, VA. This is almost an hour south of Ashburn, or about 16 minutes northeast of Midland, VA.
If you drive straight through without stopping at all, the total distance is 45 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 58 minutes.