If you're looking for a meeting place or a good stop around ½ or halfway between Murcia and Maghnia, you can try Roquetas de Mar, Spain, which is about 6 hours southwest of Murcia, or about 5½ hours north of Maghnia.
About ¼ or a fourth of the way, you can stop at Huercal-overa, Spain. This is about 2½ hours southwest of Murcia.
About ⅓ or a third of the way, you can stop at Nijar, Spain. This is about 4 hours southwest of Murcia.
If you drive straight through without stopping at all, the total distance is 481 km. 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 11 hours, 38 minutes.