It's different because it's on closed roads, so not like a Sportive at all. Nothing like it in fact.
I've ridden sportives, group rides, FNRttC (I've only done two, but are my 2nd favourite group ride, really great fun, do one if you can) and MTB events, none are as good as the RLS100. Not even the Bike New York 5 Boros Challenge, Paris Night Ride (which was rubbish) or the Cape Argus.
If you haven't ridden this ride, then nothing written on this thread can get close to describing how much brilliant fun it is, (even in the torrential rain ). Until it's been ridden those who are habitually expressing negative and pessimistic views should STFU.
Those who are doing it for the first time, who have trained, who have spent a great deal of time fund raising, who have worked hard and gone to a great deal of trouble and expense to take part, those for whom riding 100 miles is a milestone, a big deal, I say respect to you, chapeau, I doff my cap, 'ave it, well done.
You're going to have a great day.