The intercity bus system (Identified by initials, KTEL bus network) is very efficient, used by all levels of society, since Greece has almost no rail network. Modern A/C vehicles, comfortable reserved seats, footrests, departs promptly on schedule from 2 different KTEL terminals in Athens, very modest rates (example, €15 1-way to Nafplio 2.5 hr ride) . From a central Athens Hotel the simplest way to reach the Kifissos terminal (for the Peloponnese) is to taxi. I've found the "official" schedule sometimes hard to work with .. I use this one -- https://www.athensguide.com/practicalinfo/busschedulesA.htm -- hasn't been updated for ages, so the ticket prices aer not correct, but the schedule says much the same... this is for "summer season" .