BethW: Well done and THANK YOU! Having just had six months off with a posterior tibial tendon tear and (I'm told from MRI) permanent degeneration in the tendon, but being determined to return to ultramarathon running, it's really really good to hear from someone who is back running marathons. So far I've managed to do a parkrun...
Stress fracture or stress reaction often does NOT show up on x-ray, particularly initially. The very sharp pain would fit with a stress fracture. The good news is that if you REST it, really rest it, for about six weeks it should heal (I was advised to wear a shoe with a rigid sole). The bad thing is that if it's actually something else, it might not heal with rest (rest sorted my stress fracture back in 2011 but not the extensor tendonitis I'd developed at the same time - that needed physio).
As for re-starting, yes, you need to start with small distances/short times and work up again. But I worked up from restarting end of May to half-marathon by October and 50K by the following January. Good luck!
carterusm: Yes, I know - all my fault! But I think you'll enjoy it. For eating, try slowing down on your long slow runs - then there should be enough blood supply remaining going to your gut for digestion. Other than that, it's just practice and finding out what works for you. Gels may be fine for 50K but most people seem to want solid food for the longer distances.
As for me, well, I tore my posterior tibial tendon in a stupid cycling accident end of September, so my plans for this year (Pilgrims 2-day, London Marathon, Comrades, possibly Ridgeway 85) are toast. After a six month enforced rest, I have so far worked back up to doing a hilly parkrun in just under 29 minutes! So, rather a long way to go to get back to ultras, and that's assuming the tendon (permanently damaged, I've been told) holds out.