A 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.