I started running again a few years ago after a 5 year hiatus from any regular physical activity. I logged my Time, Avg/Max HR, and Calories burned for each run and there really wasn't much of a change in metrics over the 3 year period I tracked them. My heart rate was surprisingly stable and run times reduced gradually but only about a minute a year.
Maybe if I was incredibly unhealthy but I generally eat well and at my worst I've only been 20-30lbs over weight. I know that sounds like a lot but I'm 6'5" and my "ideal" body weight is 190lbs which is kind of ridiculous.