Hitting a wall during long runs

Hi team -

I have been hitting a bit of a wall at the 10 mile mark in my long runs and would love some input on how best to address. I feel strong for first 10 miles (in good control, heart rate 145-168) and then inevitably I hit some kind of wall and my heart rate shoots up into the 170's-low 180s at the same pace I have been maintaining the whole run. I get I won’t feel as fresh at the end of long runs compared to the start, but I wonder if there are things I can be doing to delay the post-mile 10 HR spike/fatigue.

Wondering if this is an indication of poor fueling/low fluids & electrolytes? Is this a fitness thing that will work itself out as I continue to train? Open to any and all thoughts!

Thank you,

Jess