This is an excellent site to give you help with this...
i would say "estaba caminando"
"The past perfect is NOT used to indicate an action which began in previous past time and is still in progress in (simple) past time; the imperfect tense is used for this purpose:
Hacía cuatro años que vivíamos aquí. We had lived (had been living) here for four years. " --taken from the link
So if I wanted to say something "had been used" long ago, that's not an action that's still occuring, right? So would it be okay to say "habia usado"?
Thank you for using the Jiskha Homework Help Forum. GuruBlue gave you an excellent site for the Perfect Tenses.
You will be primarily concerned with the Present Perfect Tense (Present of the auxiliary "haber" plus the "past participle" as in "he estudiado" = I have studied. OR The Pluperfect Tense (Past Perfect Tense) using "haber" in the Imperfect plus the past participle as in "había estudiado" = I had studied. There is, however, a Preterit Perfect Tense "hube estudiado" used mainly in literary style to indicate that the action or event had just ended. And, of course, there is a Future Perfect Tense "habré estudiado" + I will/shall have studied and a Conditional Perfect "habría estudiado" = I would have studied. You might also come across a Perfect Infinitive "haber estudiado" = to have studied or a Perfect Participle "habiendo estudiado" = having studied.
As for your question about "had been + gerund" you might see "había estado + past participle" or "había sido + past participle" however you will be most apt to use the "hacía + period of time + Imperfect" as you mentioned above.