Sein Ermatten ist das des Gladiators nach dem Kampf, seine Arbeit war das Weißtünchen eines Winkels in einer Beamtenstube.
His exhaustion is that of the gladiator after the fight, his work was the whitewashing of one corner in a clerk's office. [Kaiser/Wilkins]
His exhaustion is that of the gladiator after the combat; his labor was the whitewashing of a corner of the wall in his office. [Hofmann]
Hoffman writes as if Kafka meant to specify himself; this is certainly true, but the reader should know that Beamtenstube does refer to a specific kind of office, belonging to a clerk or a civil servant, as opposed to a medical doctor or a private eye.
Two images of struggle. It seems to me that there is more and more evidence here pointing to a fixation on struggle in these aphorisms, which bears out a similar fixation in the fiction. This one refers to the disproportionate exhaustion that can be induced by ordinary tasks, their unseen heroism, describing the relationship to the world as a struggle. It isn't that even something as minor as this is a struggle; it's that precisely this kind of thing is at the heart of the struggle. Battles are typically decisive, but a mundane task like whitewashing, while it may be done or left undone, is not historic because nothing concludes or begins with it. You whitewash now and then. The moment the whitewash is freshened up it becomes a blank canvas to be smudged and sootied all over again. In a battle, men are killed, and while more men will probably come along, those dead men can't be restored to life.
So the effort involved is the same, even if the outcomes vary wildly in significance. If we value things according to the amount of effort they cost, then this would tend to level these two things, battling and whitewashing. But if we value things according to what was won or lost, then these two things are as far apart as possible. The contrast between the former equivalence and the latter incomparability is the object of this aphorism.
On the one hand, it's a dry joke about tedious workaday chores. On the other hand, it seems to elevate, in a way, that work -- unless the intention is to compare the gladiator with the whitewasher. If that's Kafka's idea, then the point is that the whitewasher is at the same time capable of a maximum effort, just like the gladiator, and yet this maximum effort gives a result that falls bathetically short of the accomplishment of the gladiator (glory doesn't necessarily enter into the question -- the gladiator may be a monster, but the result of his fighting is death, permanent and consequential and hence unlike whitewashing). There is an equivalence and an incomparability in this comparison as well.