grammatical number - Referring to X (plural) units of Y as an "it"

I am reading The White Spider, a book on mountain climbing, and I got hung up on this passage which sounded wrong, although I can see why it isn't...

Herman couldn't be expected to hear him in that raging tempest; nor could he have come down a hundred feet of hard-won ground again, after such a struggle to climb them."

"Them" is agreeing with "a hundred feet" in this sentence, but would it be wrong to replace "them" with "it" because 100 feet represents a unit of distance more than it does individual feet? Or would "it" be preferable on the grounds that the entire noun phrase "a hundred feet of hard-won ground" should be considered the unit? Or is it simply correct as is?

The reason it stands out to me is because I commonly hear runners in conversations where the following is said:

"How far did you run today?"

"I ran about 20 miles."

"How long did it take you to run it?"

"About three hours."

In this case, 20 miles represents "the run," a single event taking three hours. The 100-foot descent seems more meaningful as a unit and not a collection of feet.

On the contrary, I would never use this logic if I were talking about eating sausage links at dinner.

"I ate nine sausage links."

"How long did it take you to eat them?"

Is it just lazy English to mistake the numerical agreement, or does it come down to the way we contextualize units?

The post script is that the book, The White Spider is translated from German.

Answer

The writer makes a hundred feet plural to emphasise that each individual foot was a struggle. It's not a single stretch of 100 feet; it's a hundred tiny journeys which have been struggled through and which would need to be undone.

The rest of the question is entirely right: when you are considering a single distance, or a single amount of money, then use a singular verb.