First of all, sorry you are experiencing this injury at the worst possible time. Been there, done that, 2017, also left knee. I had to cancel my flight and hotel and scrap the Boston Marathon then. It SUCKED.
I suggest going to an orthopedic doctor and get an MRI of the left knee and see what exactly you are dealing with. What is the pain like? Sharp or dull? Lateral or medial side of the knee? And are you sure it is PFPS/chondromalacia?
Continuing to run on it is touch and go. Don’t do anything like deep squats, going downstairs, or running downhill. If it hurts by mile 3, stop the run.
Clam shells help, so do straight leg raises (done slowly, count to 5 seconds up, hold leg up for 5 seconds, and slowly drop leg back down for 5 seconds).
I also highly recommend starting to take a daily turmeric supplement. And find some Voltaren (diclofenac) anti-inflammatory gel to use as needed.
If you can’t run on it at all but want to maintain your cardio fitness (and sanity), spinning classes (low resistance) and the Cybex Arc Trainer (not the elliptical) help a lot.
Good luck, and hope to see you at Boston.