I found a conversion page for stadia to miles.
20 stadia = just about 2 miles
Now Don has the following in his paper, which looks like the distance was quite a bit less.
Quote
Cicero’s walk across Athens, through the Dipylon Gate, and ending at Plato’s Academy provides a tantalizing route for a walking tour through the ancient city. According to Cicero’s text, he and his friends walked six stades from the Dipylon Gate while talking about various topics (inde sermone vario sex illa a Dipylo stadia confecimus), and his companion Pomponius remarks that, upon arriving at the Academy, they had just passed (quos modo praeteribamus) Epicurus’s Garden.