As the capital of the state of the same name, Campeche is well-connected by bus with many other cities throughout the region.
Buses depart regularly from Mérida, with the journey taking just a few hours. You can also find direct buses from Cancún and Ciudad del Carmen (not to be confused with Playa del Carmen).
Campeche has two main bus stations: ADO and Sur. The ADO station is too far from the center to walk, though a taxi should only cost you $50 MXN.
The Sur terminal is located about 15 minutes on foot east of the center, and this is the station you’ll want to go to for direct buses to Xpujil, the nearest town to Calakmul.
Note that you can find Mérida-bound buses at both bus stations.
Campeche also has a small airport with connections to Mexico City.