alt Hacker NewsA long race will do it just fine. AKA Runner's High.
I also do hill sprints. I try to do one [HIIT] workout a week and also some longer runs.
Hill sprints tend to be safer then sprinting on flat surfaces. Last time I did my HIIT sprinting on level ground my knee didn't really like that at all. A stationary bike (like the "assault bike") is even safer. Or you can just do burpies. People who don't run and haven't sprinted in a long time shouldn't just jump into sprinting.
I have to disagree on both points. Long races don't do this for me - that's also the point of the paper. And I have had more close calls of spraining an ankle when running uphill. The problem with hills also is that you might want to jog down after, but that's super bad for knees.