Having done this event after training 3 months I can tell you it is possible (i did it in 1.54 the first time) have done it a second time as well.
I would try and get off the treadmill and outside as much as possible- it is an undulating route the treadmill will not prepare you for this)
Your 4 miles is a good start, i would just take it steady and build up your mileage. I kept a log of each run so i could track improvements. I can't really help on the footfall issue as its not something i have experience of, defo keep an eye on it...
Perhaps slow down and concentrate on your form? plus you might just be expecting too much..i mean it is your first 4 miles you're bound to ache afterwards, go easy on yourself
i had my first ever migraine after running at the gym. One of the worst experiences of my life, i was sick, fainted, sensitive to light, neck ache and extreme pain for several hours.
Haven't had once since but i must say i was very wary about going to the gym for a while. Saying that been back several times now and nothing..so might be no connection? but i truly sympathise with fellow sufferers
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kepe promis
i'm doing it! 
