Running

I could be wrong and I hope someone will correct me if I am, but my understanding is that the only reason Z2 training is recommended comes down to the ability to train for longer and to recover faster before your next training session.

This means it's much, much more relevant for professionals doing 10+ hours per week then it is for casuals running maybe 2 or 3, if that. They do it so they can still get the benefits of more running, but be fully recovered to get maximum performance out of their smaller number of Z4/5 sessions.

If you aren't doing high intensity sessions like fast intervals or hill reps, but are instead doing all your runs at the same intensity, then making that intensity the highest you can (without injuring yourself) is probably optimal within those constraints. And Z3 encompasses that level of effort perfectly.

I'm not saying, by the way, that doing only Z3 is optimal for amateurs. Especially if you go up to 3 runs per week I think making one of those a higher intensity and then dropping the others down is probably even better. (My basic running pattern when I want a block of thoughtless training is a Sunday long run of 60ish minutes, a Tuesday high intensity interval session, and a Thursday slow, easy 40 mins.) But with just two 45 min sessions, and if you don't want to incorporate intervals or increase the complexity of your training too much, Z3 is great.