i have this burning question and hope to get some advise and guidance, i am 39 yrs old (max HR of 181). I was 154kg in Oct 2019 when i started running and today i am at 135kg.

My pace when i started was 13-15 mins/km and now i am averaging 10mins/km. When i try to push harder to pace of 9 and below, within few minutes i will hit the Zone 5 (90% of max HR) and i would not dare to continue.

The question:
1- is my HR spiking too fast into Zone 5 comparing to another person that ranges in Zone 3 doing pace of 8 mins/km related to my body weight and fitness level?
2- should i continue myself to go beyond my comfort level in zone 5 to sort of train my heart n body to adapt to higher demand?
3- does it indicate underlying health condition like narrow heart vessels forcing constricted blood flow causing the heart rate to spike to zone 5?

I am also considering going for stress test but would be good to know if the mitigation factors are known by others before so tht i could prep better for my stress test. thank you.