When I was in my 50s, I would run on a treadmill while using a heart monitor. I would take 440 splits for my pulse while running repeat one mile runs. My pulse would go higher on a seven mile run at just over 7 minute pace than it would running a mile at just under 6 minute mile pace. Sometimes I would monitor my pulse in the minute after the run, while walking slowly on the treadmill.
The pulse indicates the bodies demand for oxygen. My pulse went up slowly because I was really fit and the treadmill gave me perfect pacing. If I ran on the roads and went out too fast, my pulse would be really high and later, when I settled in, I would be going faster, but at a lower pulse rate as my oxygen demands were lower.
Right now, you have to establish base points for your pulse at varying speeds so you can monitor your improvement.