Building aerobic

Bogdan

Hi all, I want to start training for a Triathlon in the near future and I'm start with running.I had buyed a heart monitor(Fitbit Charge 2) and I have doubts how good is it.During running I try to stay at 140 beats per min and I struggle to stay in that zone even when jogging.I'm doing weights for years and I was doing conditioning training aswell but mostly anaerob training.What do you think guys?My Fitbit watch is failing or I suck at running?How should I start building my aerobic conditioning? Thanks guys!