Well, I went and did it anyway. 50km in just over 5 hours. Not many Brits on the results, but one of them was Tim Dunford who came first in the 100km version. Good effort!
Most of the climbing was tracks and fireroads, some of it up ski pistes (with a fair bit of 20% gradients and a couple of 30% parts but that could be GPS error) it spread everyone out nicely and gave time to enjoy the views of Mont Blanc and the Alpine meadows.
Dear lord the descents were scary. Not much doubletrack at all, just flat out euro-madness down just about every kind of trail I've ever ridden: mud, roots, rocks, gravel, meadows, off-camber strips of mud across black runs, more roots, roads, wet roots, stream beds and the rest. In 50 kms there were three big descents - all of them were 20 to 30 minutes of continual descending (except when struggling out of bushes).
The ambience was great - loads of people cheering and ringing cowbells, what seemed like hundreds of volunteers stopping traffic at every road junction for the valley crossings, all with a smile and a "bravo/allez/courage".
The feed stations were piled up with banana, orange, saucisson and cheese (Beaufort of course).
Although I was signed up for the 30/50km MBClassic race it was a really accessible event. We started near the back and there were people on some really ropey bikes, but all having a good go (and some of them going pretty fast too). I think this also helped me to finish in the top half of the field for the first time ever, but it felt more like the lower places of a French cyclosportive than a serious XC race - everyone was there just to dig deep, give everything and finish.
I really enjoyed it.
Edit : If anyone's interested, the routes are on this site : http://mbrace.livetrail.net/parcours.php where the full results can also be found