Why can't I keep my heart rate down while running?

I'm trying to train for a marathon, and am getting really frustrated trying to keep my heartrate between 150-165, the recommended zone for my marathon training based on age, resting heart rate, max, etc. Anyway, the annoying thing is I'm running unbearably slow and it still ticks past 165 and into the... show more I'm trying to train for a marathon, and am getting really frustrated trying to keep my heartrate between 150-165, the recommended zone for my marathon training based on age, resting heart rate, max, etc.

Anyway, the annoying thing is I'm running unbearably slow and it still ticks past 165 and into the 170s. I can keep it under 165 for maybe a mile, but eventually no matter how slow I run, it will keep trending upwards. On my last run, I ran 3.5 mph and my heart rate kept going about 165, but then I WALKED at 3.6 mph and my heart rate goes down and will keep going down into the 130s.

I figured it must be my form, but I've tried to work on that for months now and it's still not improving. Chirunning, running midfoot, like a gazelle, long strides, short strides, you name it I've tried it for at least a couple runs. Each time, my heart rate just goes up. Why is it that I can walk faster than I run, but my heart rate goes down while walking and up while running?

I'm putting in the miles, but at this rate I'll walk more than half of my marathon, which is just not how I envisioned it. Any thoughts?
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