I usually start off real easy, shooting for low zone 3, HR~145, to be honest, I don't think I could complete a Z2 ride, I just get too excited. Then I look down and see I'm at 165, then I slow back down for a minute, forget that I still have 30, 50, 80 miles to go, and see I'm up at 160 again.
Then I slow down again... and there's a short hill so maybe I push 170 to get to the top, and then I see that and slow down again, and then I end up turning into the wind... You get the idea.
In short, my pacing is pretty bad, so don't do that, however, if you keep getting out and working on pacing, you will get better. Power should give more immediate feedback, if given the option, though not something I have.
I did a century the other day and I was ~10% Z4, 60% Z3, and 30% Z2, It probably would have been more comfortable if I avoided the Z4 for sure.