I'm riding the 84 mile Reigate Rouleur on 7 July as my dress rehearsal, it has a similar course profile and climb per mile.
I've nothing planned for this weekend, 22/23 June and the remaining July weekends, if you're interested in meeting up say at Boxhill for a 40-70 mile ride, or mid-afternoon during the week?
Herein lies the problem! I'm out of town this weekend and maybe 22/23 June too, plus various things the weekends of 15/16 and 29/30. So options for anything long are looking a bit limited although I am definitely keen on 40-70 around boxhill in principle. Mid-afternoons are out with work although maybe I will just take a day off.
But basically I am banking on marathon fitness + shorter rides.
You should be fine, a summer mass sportive has got to be easier than the Dorking 100 in near zero conditions.
I can't see who you are but I certainly hope you're right!