You cannot run 14 miles at max heart rate, it's just not possible. Max HR means max VO2, except if your heart has some serious issue which prevents it from bringing the oxygen efficiently (and this could also be the reason for your collapsing, but the ambulance would probably have noticed something). And at VO2max you can run about 1 to 2 miles, that's all. So you were probably far from HRmax.
If you were at too high a VO2 for too long, what we would expect is more that your glycogen (and creatine, and everything) reserves deplete and you become physically unable to move your legs any more. That would be a local issue, not something to do with your heart.
Your problem has maybe more to do with a Sun stroke or something like that.