SUB THREE OR BUST!

Just jog 5-6 miles a day to maintain fitness. You will not lose much. If anything will keep you from running a good marathon, it will be your foot, not your training.

Be careful with combining cortisone and running. Cortisone can cause your tissue to weaken, and some runners have even had their fascia detach because of that. Ouch! In any cause cortisone will only mask the pain, but not cure you, so although it will help you get through your marathon, the PF may come back with a vengence after the shot wears off (anywhere between a few days and a few weeks from now). Keep wearing the night splint, no matter how uncomfortable it is (you will get used to it). Also, stretch your calves and achilles good after each run, and try ice massage on your heel and arch. Consider trying taping; it will essentially put your foot in a "sling" and may allow you to run without further damaging your plantar fascia. Google for "low dye taping" for some diagrams. A good PT or pod can do it for you too.

As far as training, have your tried using an elliptical trainer? It's a pretty good workout and doesn't put as much strain on your feet. You can even do "tempo run" efforts on them and get your HR in the LT zone, plus recovery is faster because it's non-impact. Also, pool running is a great non-impact workout. Your mechanics will be the same as running, but will cause ZERO stress to your foot and other joints. This means that you can do hard workouts virtually every day, recover quickly, and build as greater degree of fitness that through actual running. Boring though. I saw many of my college teammates do pool running through an injury and then be All American a few weeks later, so it does really work, probably better than any other form of x-training.

Anyway, be careful with the PF, and good luck!