Great question since there are a few of you out there that haven’t started yet. As I mentioned at the meeting, 80% or more of your training is going to be in zone 2 anyways. So, yes, building your conditioning in zone 2 makes the most sense. Since it sounds like you haven’t been training in zone 2 (thank you for being honest!!), it’s going to take some practice to understand what this pace feels like for you anyways. Then you have to ensure that you have the appropriate cadence and mechanics (both bike and run).
So yes, zone 2 focusing on good mechanics is the best way to establish the conditioning you need to get started. You can also look in Training Peaks to see where you need to be at the beginning of your plan.
Hopefully this question was a good education for anyone just getting started or still in their ‘off season.’