Get It: Push yourself hard, but not too hard. Endorphins are painkillers produced in response to physical discomfort, says Matthew Hill, Ph.D., an associate professor at the University of Calgary’s Hotchkiss Brain Institute. But that doesn’t mean your runs should be excruciating; you need to find a sweet spot where they are comfortably challenging (think tempo run).

Source: https://www.runnersworld.com/training/a20851505/how-to-achieve-a-runners-high/