AeT test query

So I did this on the treadmill. Did a 15min warm up which got me to around 130 although my Hr didn’t stabilise until 20mins @134. I ran the test no issues but weirdly the second half felt more comfortable than the first half of the hour? I was nose breathing throughout.

My start figure was 134
1st half average was 135
2nd half average was 138

Which by my maths is 3%? So top of Z2 is about 134-136?

Questions –
If I’m doing a lot of elevation on my runs I guess it will just be slow walking / hiking uphill to stay under 134-136?
Is it possible for someone’s aerobic system to take longer than 20mins to come online? The second half felt so much better from 40mins onwards. I also find when I train unless I start super super slow my Hr sky rockets… and usually I settle around 40mins in. Will this improve as my AeT does?

Top of Z2 before today was 150. Probably explains my terrible/inconsistent season so far with a lot of illness and fatigue.