Misjudged Travel Time

Five hours in buses and I'm stuck for the night in Xela (also known officially as Quetzaltenango) instead of Tapachula across the border in Chiapas, Mexico.  Buses in Guatemala don't usually travel at night so four hours of buses tomorrow will bring me to my desired destination. Then I'll look for the first-class bus station where,  if I'm lucky, I can buy a reserved seat for the overnight bus to Oaxaca.