Some friends of mine are on this: sadly, it's well beyond my capability now.
A few years ago, possibly, but that time has passed...

I did the Medio , lots of the climbs are long but steadish at a steady gradient of 5% , but anything with Devil in it's name tends to be steep but shorter . Just go easy early on and you will be fineAnother old thread revival in the sportive section....
I'm got talked into doing this....
Entered for the Gran Fondo as that's what my friends had entered but I'm going to do the Medio instead - i think that will be more enjoyable and less of a chore. 2 of our party were entered for the Fred Whitton as well, but one had a nasty appendicitis emergency just before it so only 1 did it! I generously suggested I would accompany the appendix-less one on the Medio as she thought the Gran might be pushing it a bit only a few weeks on from surgery!
Dragon Slayed - well I made it round! Git super lucky with the weather, still dry day, sunny intervals 20*C ish, couldn't have ordered it better.
My friends completed the 300k route...
Well, actually, they only Strava'd 299k so it doesn't count 😂
I'd have cycled round the carpark for a km (if I'd had the energy!) - actually I wouldn't have known as my garmin is set to miles!
Ended up with 108m for the day as we cycled up from Porthcawl digs to keep out of the carpark chaos.
Much kudos to your mates - no way could I do that distance and climbing in a day.