If you read the latest news on the website it says that the rugby club fields and surrounding areas will be unlikely to be used for parking given the weather we've had in the last few months (Mainly grassy areas rather than actual hardcore parking or tarmac)
Street parking is likely to be the option for most people going by car
Would suggest getting there a bit earlier than you intended
It's my first run at Wilmslow, I've taken a couple of junior rugby teams there in the past and access hasn't been great so can imagine it's a bit of a struggle with 5000 runners and associated marshalls, supporters etc
Not sure where the best place to park would be if you don't fancy gambling on being able to park on the street within a reasonable distance