It's a funny one isn't it? If you said 250+km with just 1,400m or so of elevation you'd ordinarily think that was pretty straightforward, especially in a group, but the pave must make it a completely different proposition.I rode the cyclotouriste event a couple of times in the 1980s.
One year it was fine, the other it rained.
The 240 km route took us (reasonably fit club riders and racers) over 9 hours.
Went over most of the famous paves, not the Arenberg though, it's a protected monument.
I remember we used cyclo cross tubs, good on the cobbles but a bit slow on the tarmac.