Hi Rob,
You are clearly someone with a good natural degree of competence. However, its no surprise that you've had injury issues if you bash out 6 x 6min miles at the end of your 90 min long run. That is suicide for a 50 mile per week runner! Could you be an individual who has been prone to overtraining in the past?
I think you need a decent weekly training plan for your level of competence. Google is full of them (sub 35 min 10K, sub 80 min half marathon etc). Alternatively I'm currently enjoying The Running Formula by Jack Daniels.
As for pre race warm up.....a couple of easy conversational miles and 4 or 5 strides seem to do it for most people. But I'd put my money on you being too quick a starter. You mention 'manic starts', but a start shouldn't be much faster than race pace. I think you probably bash yourself into oxygen debt chasing the better runners and feel awful trying to hold on.
I'd gauge your level of competence for a certain race with lower distance time trials in practice at expected race pace, and try and run races at even pace by keeping a disciplined eye on the 1/4 miles on a sat watch.
That you even jokingly suggest a 90 min warm up indicates that for all your natural ability you have very little direction with your training or race strategies. A bit of reading and research might be a good call.
As for cross country, I wouldn't worry about your pace per mile. Its often far off what you'd expect on a flat road or track.
Take care