Stubborn Stress Fracture

I have a stress fracture that is taking a very long time to heal. Here are the details: I'm a healthy 36 y/o male, healthy diet, no history of fractures, long time competitive endurance runner. Diagnosed stress fracture in lower left tibia in mid september. Showed up on both x-ray and bone scan. Most likely due to too much running. 2 weeks later I was put in walking boot for 3 weeks. No exercise during that time but standing about 4 hours a day for work. For 5 weeks after boot I only swam and occasionally rode my bike. Doc said I was good to slowly try running again after 10 weeks off. Started combination of walking/running 3 days a week. Run time never got more than 10 mins at a time with 25 min total after 2 and a half weeks. I began to have mild pain at fracture site with palpation but not with activity. I went back in and doc. said stop running again. He wanted me to bike and swim if there was pain and try light running if no pain. He also suggested a bone stimulator. Currently I'm about a week with bone stimulator. I'm doing some walking, cross country skiing, and swimming. I have slight tenderness with initial light touch but no pain with firm palpation on fracture site. Help!! It's been four months since I've run with any regularity and I'm nervous to try again. Have you heard of stress fractures taking this long to heal? I don't know if I should be doing any activity now. Is there anything I should be doing or not doing to help this along? Should I be doing the low impact stuff or go to the other extreme and get some crutches and put no weight on it? Any information and suggestions that you may have would be greatly appreciated.