since I last did it

It's fine.

CB

Hi CB!

A pronoun must always refer to a single, clear, unmistakable noun antecedent.
It, which appears at the very beginning of the sentence, has no noun antecedent at all.

According to this rule the given sentence by HFM would be incorrect. So why is this sentence fine?
I

This use of "it'' is a special case, and is very common.

HifaMoIt has been so long since I last did it.

The example is also unique in other ways.
The normal declarative version would be something like, "It has been a long time since I last did it."
To my ear, "It has been so long since I last did it!" is an exclamatory sentence.

AvangiThis use of "it'' is a special case, and is very common.

What about the second it?

mekkalompWhat about the second it?

It has been so long since I last did it.

Sorry, I meant to address that!

The second "it" definitely requires an antecedent, but it can be "located" in prior context.
The two "its" have absolutely nothing to do with each other.

Incidentally, there

AvangiThere's also a more scholarly name for it.

In Scandinavian grammar books of English it is called a formal subject but I'm not sure whether this term is familiar to native speakers. A famous American grammarian named George Curme called it "preparatory it" in his thick book entitled Syntax.

We don't know the asker's nationality and what gr