以前はタバコを吸い、かなりのヘビースモーカーでした。

、。

I used to smoke and was quite a heavy smoker.

Literal

Previously [topic-は] tobacco [object-を] smoked, considerable heavy-smoker was.

以前は (previously/in the past) sets up a contrast with the present — the implication is 'I no longer smoke.' The te-form 吸い links the two clauses. ヘビースモーカー is a direct English loanword. かなりの (quite a / considerable) modifies the loanword naturally.