Only after a couple of weeks of running about 20km, did I start seeing my resting heart rate drop to the high 40s. After that, it gradually lowered over the next 18 months or so.
Furthermore, an article about a study back in 2008 [1]:
> At the end of the 90-day study period, both groups had significant overall increases in the size of their hearts. For endurance athletes, the left and right ventricles — the chambers that send blood into the aorta and to the lungs, respectively — expanded. In contrast, the heart muscle of the strength athletes tended to thicken, a phenomenon that appeared to be confined to the left ventricle. The most significant functional differences related to the relaxation of the heart muscle between beats — which increased in the endurance athletes but decreased in strength athletes, while still remaining within normal ranges.
> “We were quite surprised by both the magnitude of changes over a relatively short period and by how great the differences were between the two groups of athletes,” Baggish says. “The functional differences raise questions about the potential impact of long-term training, which should be followed up in future studies.”
[1] https://news.harvard.edu/gazette/story/2008/04/exercise-chan...