I take your point about not wanting a parkrun "right on the outskirts of London" but, other than that, Cassiobury parkrun near Watford definitely ticks all your boxes:
Hope this helps - enjoy whichever parkrun you end up doing
These guys always get good results http://www.purplepatchrunning.com/index.php/coaching . Good luck with the GNR
I think this seems to fit the bill of what you're after! http://www.purplepatchrunning.com/index.php/26-2-team-challenge Great team event.
20 miles in a day is definitely a long run It's like extreme interval training! You will certainly get some benefit from it without having knackered your body doing 20 in one go.
From the website: "The race will be run over a new course to circumvent the flooding we encountered last year."
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