I stayed at the Athens Hilton in May last year. It was a good hotel, but could use a remodel. On our Athens map, there were two Hiltons. One called Hilton that had a footprint that looked like our hotel, and an Athens Hilton, which looked like a square and actually was our hotel. This confused us the first couple of days.
The Marriott is not within walking distance of downtown, so you will have to take the hotel shuttle or public transport.
We had a great time, but the flights were pretty long. Good Luck!
Tanya