If you're looking for a meeting place or a good stop around ½ or halfway between Manguito and Mariel, you can try Guines, Cuba, which is about 1½ hours west of Manguito, or about 1½ hours east of Mariel.
Other possible places to stop near the midpoint include San Nicolas, and Madruga.
About ⅓ or a third of the way, you can stop at Alacranes, Cuba. This is about 1 hour west of Manguito.
About ¾ or three fourths of the way, you can stop at Santiago de las Vegas, Cuba. This is a little over 2 hours west of Manguito, or almost an hour east of Mariel.
If you drive straight through without stopping at all, the total distance is 227 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 2 hours, 53 minutes.