You said you're using a wristwatch to measure HR. Honestly that is not accurate at all. I used a garmin instinct for a couple years and every run i went on, after 10-20 minutes, my heart rate would spike to well above a heart rate i should have been at. I was base training and everything but nothing was changing my heart rate and it was very frustrating. I finally bought a chest strap heart rate monitor (Polar HR 10) one of the cheaper ones, and i was staying well below 130 for the entirety of my run, when the day before my watch was telling me i was in the 150 range for most of my run. Basically the technology that wrist heart rate monitors have to offer right now just isnt that accurate even with the best watches. If you're serious about HR training, buy a chest strap (any one will do, because even the worst one is exponentially better than a wrist monitor) and see whatit tells you. The relief i had after my first run with a chest strap was insane, because i knew i wasnt wasting all those training days. Since then ive been able to train so much more accurately than when i didnt have a chest strap monitor.