Interesting. In the literature, the symptoms of myocardial bridge seem to be more heart-oriented, in that the discomfort is described as similar to that of a classic MI, with chest pain and pressure, left arm or left jaw pain, and excessive fatigue, whereas you describe a 'flash' of chest pain and then your legs fail or run out of gas. I assume you're referring to a symmetrical leg weakness?
If it's symmetrical and fast-onset, involving only leg failure, that doesn't sound like heart. It makes me wonder about a nervous system glitch.