Einer staunte darüber, wie leicht er den Weg der Ewigkeit ging; er raste ihn nämlich abwärts.
A man was amazed at how easily he went along the road to eternity; the fact was he was rushing along it downhill. [Kaiser/Wilkins]
A man was astounded by the ease of the path of eternity; it was because he took it downhill, at a run. [Hofmann]
This suggests not only that there are different ways to take the path of eternity, but that the metaphorical topography of the path is a function of the way you take it, and not the path's own fixed property.
Kafka might mean only that the downward, and hence presumably evil, way is easier than the comfortless way of virtue, but it's still the path of eternity either way. So the way the path is taken, and not the destination, is what's good or evil? Or is there a good eternity and an evil one?
That he is running shows impatience, but also a lack of resistance; when you're facing down the slope, the lay of the land almost compels you to run. You have to lean back against the grade to avoid running. People don't stage races on downhill slopes because a slope would make anyone run faster than their strength alone would permit; arguably, the strongest runner would be the one who could manage to come in last.
Taken by itself, this aphorism gives us no reason to assume that there is another, upward way. It might be that the path to eternity is always a downward slope; if that were true, then the less impetuous and therefore probably more virtuous way would be to go downwards resisting, rather than heedlessly barrelling on.
Rushing towards eternity doesn't make sense, so perhaps this is the mistake we're being warned about: mistaking eternity for clock time.