I would say see how your LR schedule goes along, but it sounds like things are improving. You have plently of time to slowly adjust. Of course, PF is a frustrating injury with a tricky backlash in regard to recovery efforts, so I understand that building the mileage/training is a real concern.
I would focus on the marathon goal with the notion that speed and finishing time are probably not going to be as expected. If this event is important to you, then prepare on the conservative side, along with some more cross-training. (strenghten, stretch, arch supports, night splints..custom inserts...cortisone shots...whatever helps).
Also, get new shoes right away and see a good sports PT for sure to cover the bases.
Do the race that is important to you...who knows about next year or other opportunities? It is true that PF may continue to be a problem, but just seek out the right combination of treatment options that work best for you now.
Wishing you every success.