Maybe you should rule out exertional compartment syndrome.

My symptoms are, within 10 minutes of starting to run, or even very fast walking, burning- REALLY bad burning, down the anterior calf muscle- so the big muscle anterior and lateral on the front of the leg, followed by toe numbness- which I pretty much never noticed because I was so distracted by the pain, but did notice it when they had me run at the ortho clinic on a treadmill, and "foot slap" which is when you are trying not to hurt the muscle, so instead of your foot landing normally, you land really flat-footed- kind of like a Frankenstein run.

Just a thought. It's very rare- but when I asked about it on the Ultra list, many people had experience with it, and had been formally diagnosed, and had the corrective surgery.

Nanci