The reason the heart rate doesn't drop is because you are training your body to run with a certain mixture of metabolic fuel, which corresponds to the heart rate. (philmaffetone.com)
In humans, dietary proteins are broken down by digestion into amino acids, which serves as metabolic fuel or other functional roles in the body. (sciencedaily.com)
And because the stress hormones themselves release and regulate metabolic fuel use (fat and sugar), fat and sugar utilization is tightly linked with heart rate through the hormonal pathway. (philmaffetone.com)