I'm a 29 year old reasonably fit man. I used to run regularly but haven't ran much for the greater part of two years now. I do my 10 minute commute on a road bike daily, usually trying to go as fast as possible. I also practice the relatively niche sport of freediving where I train to hold my breath for a few minutes and swim to depths without air. My heart rate usually drops to 38 to 42 when I'm doing a breath hold (in the air, i.e. without the mammalian dive reflex).
I picked up running again just yesterday (for a race which is in ten days), and I was surprised to see that my HR monitor reports I'm doing most of my run in anaerobic, cf. the screenshot below.
Is there a maximum amount of time one should spend doing anaerobic exercise before it gets eventually dangerous? What should I watch out for if most of my running is in this HR range?