I’m not sure if I understand your question. Your pace was the same before and after but now with a better aerobic system its much less taxing.
Were you able to run faster prior to the base training? Is 3:40/KM fast or slow for you?
I assume the pace is slow for you and you want your legs to go faster and to work your heart harder? An 8 min max rep should be high Z4 or so, definitely well above AnT–it’s in a zone and interval length where HR monitoring may be less useful or accurate, and perceived effort is better.
If speed is what you’re after and you haven’t been working speed during the past few months, or at least performing some form of sport specific strength training, then perhaps more speed work could be useful. If you just want a higher HR, or more intense interval, then adding incline may help.
Not sure if I came close to answering anything for you.