There are some good suggestions in this thread. I'm in the walkie crowd. Find yourself a good TM and spend an hour or so each day with the slope at 10% or more. It will avoid impact, assist leg strength and is better cross training than swimming or biking.
I would analyze where those stress fractures occurred. They are usually the result of over stressing the muscles that attach to the leg bones. You may have a mechanical problem requiring correction, or perhaps a shoe problem.
As for the marathon training, I'm a miserable failure at marathons, so my advice isn't worth a whole lot. But I would ramp up my overall miles without worrying about the long run. After 8 or 9 weeks I would try to extend the long run until I was up to about 3 hrs to 3.5 hrs. (two thirds to three quarters of marathon time goal) if possible. In deference to my stress fractures I would avoid speed work, except for a few striders, and possibly a semi hard portion of one weekly run, where I would try to approach threshold for a half hour to 45 min.