@Garwboy I'm also middle-aged and not great at running. I have occasionally experienced... I wouldn't call it a "high" or a "rush" but a sense of calm and wellbeing that could easily be endorphins. What it took in my case was working consistently on my fitness for months, to get to the point where I could run steadily for 40 minutes or more, without feeling like I was out of breath. Plodding doesn't do it, pushing myself doesn't do it, just keeping going at a gentle pace for a decent while