Since, "I decided" is in the past simple, and being tired happened before the decision was made, I would backshift further to:
... I decided to walk the two kilometres home, which would have been fine if I hadn't been so tired.
(decided = past simple)
(would have been = present perfect)
(hadn't been = past perfect)
It's relevant that you were already tired before you made the decision to walk and, therefore, before you actually walked.