Not trying to go one better or anything Richard but I did 2 last year a week apart and pb'd on the second, I ran both at my race pace, but on the first one the elements were very much against me wind, rain hail you name it, it came down and I wasn't happy with the time I did and so I ran another a week later although I pb'd again I still didn't hit my target, but I will next time. I hope
I would just like to add this isn't something I would recommend to anyone, like Richard says it's down to the individual.