There is no way to make up for this lost time. If you ramp up the volume of running to much in this 3 weeks you are very likely to be quite fatigued come race day. A 5 week lay off is a short period to recover from plantar fascitis. You need to be extra careful adding running back into your program and ready to pull the plug on the race if you feel the PF pain coming back. I’ve seen athletes struggle with PF for years. Be careful.