A common usage of "en" is to refer to something that was mentionned just before, so in general, it makes sense to translate "en" by "it" in english.
For example "Du temps?. J’en ai passé à écrire toutes ces lignes." would translate to "Time? I spent a lot of it writing all these lines".

Considering "J’en ai passé du temps à écrire toutes ces lignes":

In that case "en" refers to "du temps" which sounds completely useless because it is already explicit in the sentence.
The goal here is to create a redundance to put a strong emphasis on "temps". The word "toutes" also adds emphasis.
The sentence could translate to something like: "That’s a lot of time I spent writing all these lines!" to clearly show that the main focus is on the time spent.

What you said: "I spent time writing all these lines" would translate to "J’ai passé du temps à écrire toutes ces lignes" which is also grammatically correct but slightly different in meaning.

"En" is redundant in this sentence as its antecedent is "du temps". This is an accepted redundance because it is the redundance itself that gives the sentence its special power of expression. It is not used only with the verb "passer" and it expresses a variety of notions that can be the dejection, or surprise and satisfaction, or awe of who is talking or a combination of those, and always simply the emphasis of how large is a quantity referred to; these notions are motivated by the idea of the apparently excessive, or at any rate great, quantity of something that has been needed in achieving something or that has resulted from some process. It is used with various tenses.

• Il en aura fallu des années de travail pour construire ce barrage. (slight dejection (maybe) but essentially awe)
• Ils en ont essayé des marques de bière avant d’en trouver une qui leur plaise ! (emphasis on the large number)
• J’en ai passé du temps à écrire toutes ces lignes. (possible slight dejection (but not necessarily), awe)
• Eh bien ! Tu en as des images dans ta boite. ( insistence on the great number of the pictures owned, surprise)
• Il y en avait des gens sur cette plage ! (emphasis on the large number)

An important thing to keep in mind is that when speaking these sentences they will come across to the interlocutor as having the meanings explained above only if sentence stress is put on the verb or if there is an adverb modifying the verb on that adverb. The form to receive sentence stress and intonation has been put in bold type in the examples above. Dejection, awe and surprise are essentially in the intonation, which is unfortunatelly too difficult a subject for someone to be able to say anything useful in a few words; moreover, I fear I couldn’t describe this particularity to any extent in a proper enough manner. Really, I see no better way of picking up the intonations than by listening to well spoken samples.

• Il en a vraiment passé du temps à écrire toutes ces lignes.