Last season I was at 4.3 W/kg based on 95% of my 20 minute test. I know of only one road where I can go at that for a full hour without having to stop for junctions/traffic/roundabouts/lights or whatever. Becasue of this longer rides generally average significantly less Watts due to all the recorded zeros while i am stopped.

I dont see the benefit in knowing your power for very long periods. If you want to inprove your endurance then work out your endurance power based on a 20 minute test and train at that.