I already noted it's tight and therefore already suggested taking the later connection. Depending on the availability on each day, you can have multiple options at not-so-inconvenient hours, but since I don't have the time to check the entire schedule for the OP (I could possibly provide more precise itineraries if I had any clue about the date(s) in question), I'll stick with the 13th and give you the following options:



So with options including 3-, 5- and 7-hour-long layovers (the longest layover possible is actually over 10 hours) you can even go hard-core lounging and have breakfast as well as lunch (not to mention the drinks) for free at ATH if you have gold status with Aegean ​​​​​​.
Now if you don't like to get up too early and the 13:30 flight to Milos is available on your date(s), even better .