Stress Fracture

[Dok-torr]

I obviously haven't had the opportunity to actually look at your foot so what do I know?
But if loading the ball of your foot makes your heel hurt has anyone ruled out plantar fasciitis? The tight band that connects the ball of the foot to the heel helps to make the foot into a rigid platform for toe-off and can get inflamed at its insertion into the heel.
I may be doing your doc a huge disservice here but MRI's do not always help with this kind of thing....I hope you have had a thorough exam and not just been sent off to the giant magnet to get you out of the room?
The reason its important to know if your fascia is tight is because stretches can really help (foot and calf)

[Trek1500]

Unfortunately, I've had several from running, so you have my sympathy. I don't think you should rule out plantar fascitis, either. Not that that's a picnic, but you can do some stretches that will help with it.

If it is a fracture and I'm any indication of how fractures heal, it can take anywhere from six weeks on. My worst one (recently) took three months. Good luck with your recovery. I hope it's a speedy one.