First... see your doctor about it again (I assume you did before since you got a cortisone shot)

Second, I have found that just resting is not the best way to treat PF.

I have only had one mild bout with PF since I started running, so this might not be the best example to compare to, but I found that a combination of NSAIDs (I have had prescription strength stuff before, but the last bout in April I just used over the counter ibuprofen), stretching, icing and - most importantly for me - regular use of a night splint and/or a strasburg sock, led to much faster recovery from PF.

Part of PF is (as I understand it) that the damaged sheath of tissue tends to form adhesions over night or when your foot is relaxed in the constricted position. The NSAID and ice help reduce swelling which lowers adhesion formation. The gentle stretching can help break down the adhesions without re-injury.

But the night splint/strasburg sock help keep your foot flexed overnight while allowing the muscles/tendons/ligaments to relax. This allows healing to occur while the tissues are naturally extended - but not actively flexing - and the adhesions don't form. The morning stiffness/pain that occurs when you get out of bed with a case of PF is those adhesions being torn again by the act of flexing the tissues and supporting your weight. Without the night splint, I always felt like it was a "two steps forward/one step back" situation each day I was resting. Adding in the night splint usually has led to just a few days until I had great relief from PF.. and in the case of my last bout, I was able to resume running light distances in just a few days as long as I was careful to ice/stretch right after the run and then continue to use the night splint.

Plantar fasciitis.
Do the stretch suggested in the handout linked below, two or three times a day.
https://www.urmc.rochester.edu/ortho/foot-ankle/_documents/plantarfasciitisteachingsheet.pdf
Get supportive insoles for your shoes.
Runner's World says that the average case of PF last 159 days. But, I am guessing it lasts that long for most people because they do not stop running. The magazine has recommendations on cutting down mileage too, based on how much pain your have when running. You'll have to look that up. I do not remember the specifics.
One other thing: I have recently spoken with three people who found that wearing Dansko clogs during the day was very helpful.

The above advice about icing and stretching are all good. I had it in one foot and it took a full year before I could wake up and walk without pain. Once it got healed I started the run/walk program again, but then... I broke my ankle.

Dang.

But now I'm walking again, weak ankle (probably a year until the ligaments are all strong enough to be back to 'normal'), and the bottom line is to take it slow, stretch, and remember that sometimes the slow path to healing is the strongest path.

Good luck.