Disclaimer: I myself am a mediocre female runner. My only claim for why I have this much to say about stress fractures is that I have had 2 within a span of 13 months, and as an aspiring medical student, I've collected a lot of information about stress fracture experiences from all of my (more accomplished) runner friends as well.
That said, it's hard to say without knowing more about your background, but here are some things I would look at, based on the experiences I've had with two stress fractures (L navicular, R femoral neck occurred 7 months after return to regular training) and also observing the stress fractures/treatment protocols of team mates and friends (ranging from mid-pack DIII to All American/sub-elite):
1) Treatment of fracture. Be more conservative than what the doc tells you to do. There's actually a surprising amount of variation in treatments prescribed by orthos. I've seen the best results from 4 weeks of conservative care (crutches if there is pain from daily activities, limited walking and standing if no pain) followed by 4 weeks of no running but more normal activities, followed by a very gradual return to running.
2) Return from injury... I agree with running doc that sometimes you need a significant period away from running and then start really gradually. Also relevant are... avoiding weight gain during injury, and very low impact cross training (think, aquajogging or swimming). The heavier and more out of shape you are, the more the stress on the newly healed bone. The fastest return to competition I've ever seen came from someone who aqua jogged 2 hour a day during her injury.
3) Underlying medical issue. If you truly mean that you run low mileage (ie. less than 40 mpw) its highly likely that there's a significant underlying issue, biomechanical, or if you are female, a hormonal imbalance or female athlete triad issue. In my opinion, your average runner (healthy, non-over/under weight) is not so fragile that 30-40 mpw should result in a stress fracture. I think that too many runners get advised to scale back because "their bodies just can't handle more than X" miles. This may be true for some, but I would get everything checked out by a reputable doctor who has worked with competitive runners before. Also, be sure to cover all of your bases. I've known people who thought they were healthy and deficiency free who turned out to have significant bone density loss.
For me, the thing that changed everything was a visit to a physiotherapist, who corrected a few things about my stride (the main change was hip carriage and heel/mid-foot --> toe strike). I had tried to correct mechanics on my own before, but realized that it is nearly impossible to do correctly without someone observing you and giving specific feedback. Prior to this, I had been injury prone for years before finally getting my 2 stress fractures. Desperate to be able to run, I had reduced my mileage from 70 to 30, and reduced intensity to basically 100% easy jogs, and took 6 months completely off after stress fracture #1. NONE of this helped at all until I corrected the biomechanics. Incidentally, correcting the mechanics has also allowed me to run at faster paces feeling better than I ever have.
Based on what I've observed, high mileage folks with stress fractures tend to be fine with minor adjustments such as shoes or an extra rest day, but low mileage folks with repeat stress fractures tend to have something weird going on medically or biomechanically.
Note of caution though if you do want to make changes to your stride... don't try this at home! you can do a lot of damage (including more stress fractures) by changing things pell mell. There are a lot more than just where your foot hits the ground that determines the impact felt by your joints and bones. Find a physio who has experience with competitive runners, and when you practice, don't overextend yourself, because when you are tired, your form falls apart and you are re-inforcing bad habits.
Best of luck!
