Distance is being used as a metaphor for accomplishment. Think of it as "We have to work twice as hard to get half as much done."

In the sentence, "We must work twice as hard to get half as far," the phrase "as far" means the same level of achievement or progress as before. This sentence implies that achieving a certain level of success or progress requires a certain amount of effort. To achieve the same level of success with less effort, one would need to work twice as hard as before. This suggests that progress is not always linear, and it can be difficult to maintain the same rate of progress without a significant increase in effort. Overall, the phrase "as far" in the sentence refers to a level of achievement or progress that has been previously attained or is being aimed for, and it emphasizes the importance of hard work and effort to maintain or surpass that level.