if I have a heron at a battlefield and I use discipline, and my opponent reacts with engarde, and I have no runes up, can I play a defy for no cost using the heron?

No — the discount isn't live yet, so you'd need 1 Energy + 1 Power in your pool to Defy.

Heron's trigger fires when your first card completes resolution, not when it's played (CR 420.4.a; FAQ #11762 says the discount applies to the next card played after the ability resolves). While En Garde is still on the chain waiting to resolve, your Discipline hasn't resolved, so no trigger has fired and no discount exists — Defy's cost is still [1] + [A].

Waiting doesn't help either: En Garde sits above Discipline on the chain, so it resolves first. The only window to Defy it is before Discipline resolves, which is exactly the window where the discount hasn't been created. And note the discount is [2][A][A], so if it were already pending it would indeed pay for Defy entirely — the problem here is purely timing, not the discount amount.