Behind schedule ? pity.... what kind of schedule ,how much time DO you have.?
From Guatemala City the really scenic route would be the Panamerican west through the Mayan Highlands along Xela,Huehue' and to Mexico border at La Mesilla , then on through Comitan,S.Cristobal de las Casas. See the sights and from there if you are desperate you can hook into the toll routes to Tuxtla Gutierrez , north to Mex 180 near Coatzacoalcos and onward and onward -Ptoll to Puebla,DF, Guadalajara and Mex 15 up the coast to Arizona.
If time permits you could visti Sololoa, Chichicastenango and see about following the libre routes in Mexico , through the mountains of Chiapas at least, maybe even take Mex 190 through the spine of Oaxaca ,state and city.
But you gave no time span you have to play with. Schedules are a bitch....avoid if at all possible.