On time and your trip
While I havn't been the routes your discussing I did just drive down to Costa Rica on the Pan Am from the states.
General time guidelines to consider, a border crossing can take anywhere from a half hour (rare in my case) to 5 hours. I seldom have been able to put in more then 300 miles a day down here, and thats driving all day with the light. Any big city is going to be time consuming and more dangerous in ways, and the chances of getting lost and taking alot of time to cross are good unless your good at the language and your lucky.
Anyway that should give you an idea of time and distances. Down here you just can't plan on making great time. In Guatemala I left at 6 in the morning one day and arrived in El Salvador at 9 at night, maybe covering 2oo miles or so total. Between 2 hrs at the border and the road in Guatemala being closed for almost 5 hours, it was slow indeed. Driving at night while possible, is not recommended, if you do, 40 to 45 should be your max speed, and one unseen pot hole could end your whole trip.
The horses and stalled vehicles in the road with no lights are just a bonus to night travel that can be dealt with at a slow speed.