I don't know what your terrain looks like, but from where I sit you and Primoz must be smoking something. Around here, 50k of singletrack is a full-on all day endurance event.
Maybe not smoking is the key here
But thing is, we are not all same, and while I can say, I'm still in pretty good shape, I'm not anything near racers, yet I can still comfortably climb at 800-900m/h if climbs are long (1-2h). I still prefer to ride single trails, but it's pretty much mission impossible around here to get 50+km loop consisting only of single trails, so yeah there are normally some gravel roads and sometimes also some asphalt in between, but normally my rides are 70-80% on singletrails. But for me, average speed depends more on amount of climbing then on amount of single trails. I can easily ride 20+km/h average on single trails, while there's no way for me to have 20km/h average on 50km loop with 2500m of ascend in it.... even if it would be all on US style fire roads (gravel roads in Alps are "a bit" less polished
.
PS: Sella Ronda Hero (just as I'm currently having fun on mtb around these places) is 80km, has 4000m of ascend, and it goes up mostly on old first World war roads around Val Gardena passes, and for down, most of it are pretty damn rocky (I never made whole 80k loop, but made few times "short" 60km loop but never during race, and most of those trails are too rocky for xc bike to be fun) single trails, and winner this year needed 4h 30min, which makes it almost 18km/h average