Running

Press the arch down on the ball?

I roll the golf ball about so it touches all parts of the bottom of my foot pushing down quite hard, not just the arch. Doing this mindlessly for a couple of hours a day was enough to get rid of my pain after a few days.

No idea if I need to keep doing it to stop it coming back, but the golf balls are still under my work and home desks and I keep playing with them with my feet (both feet despite only having the original problem in one of them).