What is the function of the first " it"?

(Cambridge Dictionary) worth it
enjoyable or useful despite the fact that you have to make an effort
♦ It was a long climb to the top of the hill, but the view from the top was worth it.

(lexico) worth it           phrase, informal
Sufficiently good, enjoyable, or successful to repay any effort, trouble, or expense

(Free dictionary) worth it

1. Worth the cost or work involved to attain it; having benefits that outweigh the costs.
   ♦ Getting up early to jog is tough, but it'll be worth it when you can come home from work and not have to worry about exercising.
   ♦ This tool set cost me an entire paycheck, but it has been worth it—I've already used it for six different projects.
2. Worthy of something; deserving of something.
   ♦ I know the ring is expensive, but Jane is worth it. I want her to have exactly what she wants.

According to these sources the interpretation provided by user Yosef Baskin¹ appears to be the relevant one; "worth it" has become a set expression.

¹ _{The first part of this interpretation, only (worthwhile); the example is an error if read—as I think it should be—in the light of the second definition in the preceding source (which is "1c" in the SOED entry (see below)—Of a person: having a certain (usu. specified) value or status, in respect of possession, property, or income; possessed of, owing.}

So, the following is one possible rendering (and, of course, the first "it" stands for "the job").

• This job involves a lot of hard work but it [the job] is useful despite the effort you have to make.

(OALD) important, good or pleasant enough to make somebody feel satisfied, especially when difficulty or effort is involved
worth something [family of set phrases, "idioms" in OALD]
♦ Was it worth the effort?
♦ The new house really wasn't worth all the expense involved.
♦ The job involves a lot of hard work but it's worth it.
♦ The trip was expensive but it was worth every penny.

(SOED) 5 Sufficiently important or valuable to be an equivalent or good return (for something); such as to justify or repay; bringing compensation for.
♦ B. KING The crown of France was well worth the trouble of hearing one mass
♦ R. BROOK A blasphemy scarce worth me saying, A sorry jest.
♦ Forbes An engaging .. spy thriller well worth anyone's time.
♦ Investors Chronicle The extra return .. is ultimately not worth the risk.

Phrases […] worth it colloq worth the time, effort, etc., spent. worth one's or the [archaic] while

According to these two additional sources, observing that nothing keeps one from using the pronoun "it" as the "something" to be specified, and noting the telling example in the first one of the two (fourth example), while keeping to the probably more precise definition found in "5" of the second of the two, the following rendering appears also possible.

• This job involves a lot of hard work but it [the job] is valuable enough to be a good return for it [the hard work].

It becomes clear, I think, that user Bold Ben's interpretation is just as likely a possibility; this then makes the construction disagreeably ambiguous. One can rest assured, however, that from the registers given in lexico and The SOED, in a formal context, only the latter interpretation should prevail, while in an informal one the choice might not be so easy to determine.

_{Note: Apparently, there is a contradiction between the Free Dictionary and The SOED as 2 in the former is a set phrase (idiom) while the corresponding 1c in the latter is a regular meaning.}