If you're looking for a meeting place or a good stop around ½ or halfway between Ashburn and Monrovia, MD, you can try Canavest, MD, which is about half an hour north of Ashburn, or about half an hour southwest of Monrovia, MD.
Another possible place to stop near the midpoint is Potomac Gateway.
About ¼ or a fourth of the way, you can stop at Goresville, VA. This is about 15 minutes north of Ashburn.
About ⅓ or a third of the way, you can stop at Scattersville, VA. This is about 18 minutes north of Ashburn.
About ⅔ or two thirds of the way, you can stop at Ballenger Heights, MD. This is about half an hour north of Ashburn, or about 18 minutes west of Monrovia, MD.
About ¾ or three fourths of the way, you can stop at Frederick, MD. This is about half an hour north of Ashburn, or about 15 minutes west of Monrovia, MD.
If you drive straight through without stopping at all, the total distance is 41 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 53 minutes.