Mixco Viejo is around 60km from Guatemala, but the road is very slow and you can expect to take around 2h30 to get there, by car. Take Calzada San Juan, and pass through San Pedro Sacatepéquez and San Juan Sacatepéquez. The road is straight forward, but signalization is almost non-existent.
There's an admission fee of 5Q for Guatemalans and 50Q for foreigners.