Yep.... will be aiming for a bigger distance goal next year :-) The legs are ok, the calf feels pretty good too.
I used to run lots in zone 3, and like you it felt pretty comfortable and natural. Dropping down to zone 2 felt really weird to begin with, but after a while it felt better Now when I'm in zone 2 it feels like I can run forever. I think the body adapts to what ever it is used to doing, regardless of whether that pace is easy or not.
When I did the runs at zone 3, I always felt like I had done a workout, and I though that was a good thing. Now I stick to mostly zone 2, I don't feel tired/stiff afterwards, and I'm quite happy to run 2 or 3 days in a row (and its the reason I've been able to massively increase my weekly distance.
A question.
Look at your average pace for your recent (ish) half marathon race, and then compare that to your average training run pace (not the parkruns). Is the pace very similar or markedly different?