I've had plantar facia for about 17 years. Its caused by the foot arch being too flat, so stretching the plantar tendon. The cure is an inner sole warmed to make it extra flexible, then pressed against the underside of the foot whilst in the natal position ( the position at birth, which is simple to achieve ) The inner sole then firms to hold your foot in absolutely the correct postion at all times. The set is not rigid, the inner sole remains flexible around the correct position.The effect of it is to correctly arch your foot, which in turn puts your knees, hips and backbone in their best positions.
I was very lucky to see someone who's very good at treating all manner of foot problems. It was not expensive ( two visits at about £20 each, and inner soles at about £80 ).Todays costs will be more .
One of the things to avoid for any injury is more bruising, which can be caused by cortisone injections, and hard manipulation of an injury! There are special treatments to draw the excess fluid out, speeding up the repair process.
To finish. I was in such pain, I could not walk more than 200m., and had to stop work. After fitting the inner sole, I was quickly able to resume work, and a few weeks later could do everything, including running 4 miles or more.That was many years ago, and I can still run ( yesterday 13.5km ). Cured!!