The thought of the SR aka Marcus ride has occupied my mind and the 'let's do the easiest' springs to mind, but even the easiest in hard!
Going way OT here - but at least we're talking about my favourite topic now - me!
I've ridden what is probably the hardest 600km event in the UK (Pendle) and what is certainly the hilliest of the Super Randonnees (15000m of ascent in the 600km).
The Pendle is a brute - 10000m of ascent in the 600km and 40 hours to do it. There's not much opportunity for sleep and you're always watching the clock. It's a hard ride for sure, the hills combined with atrocious overnight weather in 2013 made it a memorable event!
Whilst the Super Randonnee was very demanding and I wasn't prepared for the cumulative effects of the climbing (it is, in effect, a substantial power output for between 1 and 2 hours, relax for 30 minutes, substantial power output for 1 to 2 hours, relax for 30 minutes repeated 5 or 6 times a day), the relaxed time limit of 60 hours for that one enabled me to stop in a hotel for 6 to 7 hours each night.
So the time pressure is far less on a Super Randonnee, but you'll need that rest as the days will half kill you!
They are really fast tours, very hard fast tours. The climbing is completely different to our UK style climbing; I'd have probably found it easier this season having done all my training based around the 24hr (compared to the 2013 season where it was all about going up hills) as I was trained to give a constant power output for (very) long periods of time this year.
They are very achievable for anyone who has decent legs on them - the riding is very challenging (in terms of ascent, day 1 featured just under 7000m of climbing in the first 190km including two passes of over 2000m) but the time is there to recover a little ready for the next day. None of them look easy, you are right, but they are an achievable goal for sure. I found the Pendle a little harder on balance, but that's because of the time ticking away!
I do plan to do more of them as I love the big mountains. I chose the Pyrenean one partly to recapture a tour of my youth, partly because it was the hardest of the lot, and partly because it features more famous climbs than the others (starting on Tourmalet and finishing with Aubisque). I've some plans involving some of the others - just need the time, next year may be better as my mega-project I am working on will be over and done with.
But I would totally encourage any AUKs to give them a go and report back on some of the other ones (they all look fabulous!).