As a proof of her fitness may serve the fact she often rode to Berlin and back from Potsdam in one day, at a time when this way was much longer and very uneven, so really lasted eight miles.
Reminder for cahn that the German mile of the 18th century was approximately 5 of our miles
For illustration see this map from 1775, which shows the roads between Potsdam and Berlin at the time and includes a handy mile indicator. It's indeed roughly four miles one-way, so I'm not quite sure what he's saying either, as the round-trip was eight miles. (I mean, this map is from twenty years later, but I can't imagine that the roads changed all that much in between.)
ETA: Possibly like Selena said - expansion of both towns led to shrinking distance between the outskirts and that's why he's clarifying the distance. But he definitely means round-trip then.
Re: "Anekdoten, die wir erlebten und hörten"
Reminder for cahn that the German mile of the 18th century was approximately 5 of our miles
For illustration see this map from 1775, which shows the roads between Potsdam and Berlin at the time and includes a handy mile indicator. It's indeed roughly four miles one-way, so I'm not quite sure what he's saying either, as the round-trip was eight miles. (I mean, this map is from twenty years later, but I can't imagine that the roads changed all that much in between.)
ETA: Possibly like Selena said - expansion of both towns led to shrinking distance between the outskirts and that's why he's clarifying the distance. But he definitely means round-trip then.